Gordon Brown last night faced a growing Labour revolt over his handling of the wildcat strikes, which MPs warned could fuel far-right groups such as the British National party.
The protests have touched a nerve in the labour movement. The strikers' appropriation of Mr Brown's slogan of "British jobs for British workers" has been hailed by the BNP as a victory for its brand of nationalism.
Union leaders sought to distance the walkouts from the far right yesterday, saying the construction workers were shunning BNP support. "The unofficial action taking place across the UK is not about race or immigration," said Derek Simpson, joint leader of Unite. "It's about class. It's about employers who exploit workers regardless of their nationality."
But Labour MPs warned that ministers' apparent dismissal of fears about jobs going to overseas workers risked exacerbating a wider failure to address concerns of the white working class.
They highlighted Peter Mandelson's exhortation in the Lords to "keep focusing not on the politics of xenophobia but the economics of this recession".
Jon Cruddas, the unofficial leader of the Labour left MPs, warned: "If we're simply caricaturing this [dispute] as being about protectionism and xenophobia, we're completely ignoring some material issues in this and, in doing so, we hand over what this is about to more pernicious forces."
Mr Cruddas told the FT: "Trite comments about protectionism or xenophobia do not address the whole question of equality [for British workers] before the law. If the Labour party can't grasp this and articulate that, then other forces will."
The motion "deplores" the use of foreign workers and calls for additional power stations to be "built by companies employing primarily British labour". Political opponents have attacked Mr Brown for indulging in populist "British jobs" rhetoric.
David Cameron, the Conservative leader, last week accused the prime minister of championing globalisation on one hand while "on the other, he borrows this slogan from the BNP".
But some Labour MPs believe the problem lies less in the slogan than in the fact that the government has not implemented it. "Failure to deliver British jobs to British workers could wipe Labour out," said Frank Field, the maverick former minister. Labour must get a grip now, otherwise their actions will feed the BNP."
Mr Field's warning reflects a party concern that the government's failure to engage with the poorest white voters is creating a vacuum for the far-right to fill. Hazel Blears, the communities secretary, last year urged the Labour party to "recognise that where the BNP wins votes, it is often a result of local political failure".
MPs fear that European elections in June could hand the BNP its first seat in the European Parliament, potentially from a region including Ms Blears's constituency.
But polling experts caution that such fears should be seen in the context of the fringe parties' failure to make inroads into mainstream politics. John Curtice, professor of politics at Strathclyde University, said that the BNP and UK Independent Party have been "there for . . years but it's not obvious that their position now is any stronger than it has been before".