Former Tory MP Nadine Dorries, who has switched to Reform, insisted that two-party politics is over following the by-election result.

In a post on X, she said: “For Reform to have come 2nd from nowhere and to increase vote share by 34.2% in Wales is astounding.

“Plaid have had a full operation on the ground for many years, Reform arrived from nowhere.

“Labour -34% … this will send huge shock waves through No 10. Two party politics, it’s over.”

The Government understands people are “disappointed” with the pace of change, and is determined to go “further and faster” in delivery, Downing Street said after Labour’s by-election loss in the Welsh Parliament.

The Prime Minister’s press secretary said: “By-elections are always difficult for incumbent governments.

“This one is no different, but we are determined to show the people of Caerphilly and working people across Wales the change the UK Government is delivering hand in hand with the Labour government in Wales.

“(We are) determined to go further and faster, understand people are disappointed with the pace of change, and that is what the Government is relentlessly focused on delivering for working people in Wales and across the UK.”

A Reform UK Wales spokesman said: “The Labour Party is finished in Wales, and the Conservative Party has gone the way of the dodo.

“This is a historic realignment in Welsh politics.

“Next year, the choice before the Welsh people will be between open borders madness with Plaid Cymru, or common sense with Reform.

“We are fighting to win the next election, to enter Welsh Government, and to ensure that our left-behind communities will be left behind no longer.”

The Reform UK leader said: “At the start of polling day, I thought that we would get 12,000 votes and we did. I thought that number would be enough, but it wasn’t.

“The total collapse of the Labour vote to Plaid was to a party that people know well and to a popular local politician. The Senedd elections next year are a two-horse race between Reform UK and Plaid Cymru.”

Former Welsh Conservatives leader Andrew RT Davies said Plaid Cymru’s victory in the Caerphilly by-election should be a “wake-up call” for unionists.

Mr Davies said: “Separatism is corrosive, with the aim breaking apart our nation and no regard to its consequences on the living standards of the people of Wales.

“Plaid have escaped the scrutiny subjected to other parties for too long, portraying themselves as a harmless protest vote while they pursue their extreme ideology.

“The separatists’ victory in Caerphilly must be a wake-up call and the onus is on those of us who believe in the Union to show the people of Wales the real Plaid.”

Rhun ap Iorwerth, leader of Plaid Cymru, said: “I think Keir Starmer came in with a whimper.

“I predicted at the time that people would become disillusioned with his Government very soon. In fact, I said they were becoming disillusioned with it before he even formed it.

“And then he goes and cuts the winter fuel payment, and then he goes and refuses to lift the two-child benefit cap, and then he refuses to engage on issues that are important to us in Wales.

“On devolving the Crown Estate on a fair funding model, on giving us what’s owed because of HS2, and then he goes and lets down Port Talbot.

“And all the time Welsh Labour do nothing to stand up for the people of Wales.

“It’s a catalogue of reasons why people now have recognised that it’s time for Labour to go, and it’s Plaid Cymru now that’s ready to replace them as a government of Wales.”

The latest Techne poll has Reform UK in first place on 29% (-1), ahead of Labour on 19% (-1), the Conservatives on 18% (-1), the Lib Dems on 16% (+1) and the Greens on 12% (+3).

The Conservatives will desperately hope their new willingness to announce policies will allow it to power past Labour. They will dread the possibility of falling behind the Lib Dems.

Speaking in Caerphilly after Plaid Cymru’s by-election victory, winning candidate Lindsay Whittle said “the world is watching Wales”.

He said: “Yesterday the people of Caerphilly created their own history and we put Caerphilly on the map firmly.

“I have received messages of goodwill, not only from all corners of Wales, but from Scotland, France, Australia, and Spain and Canada.

“And that is a mark of how important yesterday was to Wales and the world, because now I believe the world is watching Wales and watching an emerging nation start to control our lives again.”

This SNP sees Plaid Cymru as a “sister party” and this is how Pete Wishart, its deputy Westminster leader, reacted to the result: “The historic combination of an SNP majority in Scotland and a Plaid Cymru victory in Wales is the perfect chance to beat Nigel Farage’s Reform Party and remove Keir Starmer at the very same time – you’d be hard pressed to find a better two for one offer.

“And that’s exactly the offer and the opportunity that will be on ballot papers in Scotland and in Wales next May – achieving that historic result would have the most powerful impact on a broken Westminster system that is so far removed from the priorities of our peoples.”

Reform UK MP Lee Anderson admitted to being “disappointed” at the second place result but the party has no intention on giving up on South Wales.

He said: “The Labour vote collapsed to 11% after being in control for over 100 years. Yes we know a lot of Labour voters voted tactically for Plaid to stop us winning.

“That’s politics and no complaints here, we have to win in the system we operate in. Yesterday was first past the post so tactical voting works.

“Next year’s Senedd elections are proportional representation so if Labour voters vote Plaid again it could see the Labour Party wiped out altogether. We polled 35% so we are in a good place in Wales.

“We will not give up on our Welsh cousins and a huge well done to Llyr Powell and our great team in Caerphilly. We will be back.”

There is grave concern in Scotland that oil and sea jobs are vanishing faster than clean energy positions become available as Labour pursues its net zero ambitions. There’s a major report out now and the response from Reform UK and the Tories is incandescent.

The brutal truth is unpopular leaders with unpopular policies lose elections – even in their heartlands, writes chief political commentator David Williamson.

The party must wake up to the reality it has unpopular leaders and unpopular policies – and this means it is on course to suffer further disasters at the polls.

This loss is much more important for Labour than the upcoming result of the deputy leadership election.

Caerphilly had voted Labour in every Westminster election since 1918 and in each election to the Welsh Parliament, the Senedd, since its founding in 1999.

But now the seat, which was once held by former Welsh Secretary Ron Davies, will be represented by pro-independence Plaid Cymru.

This reveals the depth of anger at Sir Keir Starmer’s administration in London and the frustration with the Labour-run Welsh Government in Cardiff.

People did not just sit at home and refuse to vote Labour; they made the trip to the polling station and dealt the governing party a humiliation which will be remembered for generations.

Read the full analysis here

Conservative Party deputy chairman Matt Vickers said the Government is “disastrous” and the by-election result is people’s way of sending a message to Labour.

“People often vote to send a message in a by-election,” he told Sky News.

“And people have told Labour that they’re not happy with them. They’ve sent that message using the by-election,” he added.

Mr Vickers said the Conservatives had a “really tough” election last year and are “learning the lessons that the British people have given us”.

Labour will “redouble” its efforts in Wales ahead of May’s Senedd elections after Plaid Cymru took a long-held seat in the Caerphilly by-election, a minister has said.

Cabinet Office minister and Torfaen MP Nick Thomas-Symonds told LBC that when he was knocking on doors in Caerphilly, “what’s been coming through to me very strongly is a frustration with the pace of change”.

He added: “People want us to go faster with the pace of change.

“We’ve had thousands of conversations on the doorstep. We’ve listened. We treat the result with humility. We will then act upon that, redouble our efforts in the six months before the Senedd election.”

A minister said Labour has a “very tough fight” on its hands in the Senedd elections next May after its defeat by Plaid Cymru at the Caerphilly by-election

Cabinet Office minister and Torfaen MP Nick Thomas-Symonds told Times Radio: “I certainly accept, and the First Minister – Eluned Morgan – has accepted, that we have a very tough fight on our hands for next May.”

He said Labour’s loss was “disappointing” and pointed to a “fractured political landscape” in the UK.

“We will listen to the thousands of conversations that we had in Caerphilly about speeding up the pace of change.”

Reform chairman David Bull said he was “incredibly upbeat” and “very bullish” following the result of the Caerphilly by-election.

Asked for his view of the election result as a whole, he told BBC Breakfast: “In some ways disappointing for us, but actually it’s an amazing result on another hand, because actually we’re only four years old as a party.

“At the last election, we got 1.7% and this morning we got 36%. That’s a meteoric rise for us, and I think actually pretty unprecedented in modern political history.”

Dr Bull referred to the “decimation” of Labour and said the Conservatives were “wiped off that electoral map pretty much completely”.

Polling expert Sir John Curtice said the result of the Caerphilly by-election showed Labour was in “severe trouble” in Wales after Plaid Cymru won the Senedd seat.

He told BBC Radio 4’s Today programme: “It does suggest that Plaid are now well placed to provide Wales with its next first minister.

“Reform will be disappointed at coming second with 36% but I don’t think we should run away with the idea that this, in any way, suggests that Nigel Farage’s bubble is burst.”

Sir John said Reform UK’s result is “pretty consistent” with how Mr Farage’s party has been polling generally.

“It’s just that it isn’t going to be strong enough to win an election if you’re facing a strong alternative, which in this case was Plaid, whereas of course in the county council elections earlier this year that wasn’t the case.”

In a statement after the Labour defeat in Caerphilly, First Minister Eluned Morgan said: “This was a by-election in the toughest of circumstances, and in the midst of difficult headwinds nationally.

“I want to thank our candidate, Richard Tunnicliffe – a good man who stood because of his desire to serve his community.

“I congratulate Lindsey Whittle on his victory tonight.

“He returns to the Senedd, continuing his many decades of elected service to people in Caerphilly.

“Welsh Labour has heard the frustration on doorsteps in Caerphilly that the need to feel change in people’s lives has not been quick enough.

“We take our share of the responsibility for this result.

“We are listening, we are learning the lessons, and we will be come back stronger.”

Lindsay Whittle, the winning candidate for Plaid Cymru in the Caerphilly constituency, said the victory marked a “new beginning”.

In a speech following the result, he said: “I hope that this will be now an exciting time for politics in Wales.

“I’ve been absolutely heartened by the number of young people who’ve been involved in this campaign.

“Listen Westminster, this is Caerphilly, and Wales, telling you we want a better deal.

“We are at the dawn of new leadership, we are at the dawn of a new beginning and I look forward to playing my part for a new Wales, and in particular for the people of the Caerphilly constituency. I thank you with all of my heart.”

Plaid Cymru has won a fiercely-fought Welsh Parliament by-election, with Reform UK second and Labour a distant third.

Plaid took 47% of the vote, with Nigel Farage’s party on 36% and Labour on just 11%.

It is a bitter blow for Sir Keir Starmer’s party, which had held the seat since the Senedd was first established in 1999.