LABOUR’S historic by-election disaster in Caerphilly is an “existential threat” to the party’s future, left campaigners warned today, as anti-racists celebrated Plaid Cymru’s victory over Reform.

Unions and anti-racism campaigners welcomed the Welsh nationalists beating Nigel Farage’s party to second place in the Senedd race as they blasted PM Sir Keir Starmer for mirroring the latter’s anti-immigrant and austerity politics.

Momentum co-chair Sasha das Gupta said: “The drop in Labour support in traditional heartlands can only be described as an existential threat to the party’s future.

“This is a damning indictment of a Labour government that refuses to tax the rich properly and has refused to genuinely reverse austerity.”

Cabinet Office minister Nick Thomas-Symonds admitted that the result left Labour with a “very tough fight” on its hands ahead of the Welsh Parliament elections in May.

But he denied Labour Senedd member Alun Davies’s claims that Westminster Labour’s anti-refugee rhetoric and its use of “the language of Reform” were to blame.

He pointed instead to a “fractured political landscape” in Britain, but polling expert Sir John Curtice said that losing its 107-year grip over the hitherto safe seat showed Labour was in “severe trouble” in Wales.

Plaid Cymru’s Lindsay Whittle took home 15,961 votes in Caerphilly, beating second-placed Reform UK’s 12,113, and leaving Labour’s Richard Tunnicliffe trailing behind on just 3,713 votes — less than a third of the 13,289 his party won in 2021.

A jubilant Mr Whittle said: “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.”

PM Sir Keir Starmer was accused of “destroying” the party by grassroots Labour members.

Welsh Labour Grassroots co-chairman Dylan Lewis-Rowlands said: “It is clear that Starmer is destroying the Labour Party. When seats like Caerphilly, held for 100 years, were at real risk of going to Thatcherites, you have to ask why.

“Starmer and co have spent too long ‘changing’ the Labour Party and not enough time representing it. From dodgy selections to misleading charts, this election has been a disaster from the get-go.

“And Starmer and his cadre are to blame.”

Stand Up to Racism (SUTR) campaigners celebrated a hard-won street campaign that helped see off Reform. An SUTR Valleys statement said: “Reform UK threw everything they had at this by-election — and they still lost.

“The defeat of Reform is a major victory for everyone committed to opposing racism and intolerance. The significance of this outcome cannot be overstated.”

Campaigner Joao Felix added: “We were told we did more than most political parties to contribute to the result of this by-election — a testament to the power of anti-racist, anti-austerity, pro-working class campaigning.”

TUC Cymru general secretary Shavanah Taj meanwhile welcomed the result as “good news for working people and progressive politics.

“It shows voters understand the real threat to our communities in Wales is underinvestment, not immigration,” she added.

“The trade union movement will continue to do what we do best — we will listen, we will organise, and we will keep fighting for a Wales that works for everyone.”

The victory for Plaid Cymru comes in the run-up to a vote on the Welsh government Budget by its minority Labour administration.

Labour has led the Welsh Parliament since the devolved administration was first established in 1999, but recent polling puts Plaid Cymru and Reform UK as the two biggest parties in Wales next year.

Richard Burgon, secretary of the Socialist Campaign Group of MPs, said that the historic loss “is the direct result of the disastrous direction taken by the national Labour leadership — from the mess around winter fuel payments to cuts to disability support.

“To rebuild our support, Labour must start governing with real Labour values.

“The cost-of-living crisis remains the biggest issue in British politics. Yet instead of focusing on that, the Labour leadership has been obsessed with imitating Reform, and Labour supporters are abandoning the party in droves.

“The Labour leadership must drop its failed approach and use the upcoming Budget to tax the very wealthiest and fund urgent action for families squeezed by soaring food prices, rising energy bills, and unaffordable housing costs.”