Gordon Brown faced further threats to his faltering premiership today as a Labour MP and a union leader joined a chorus of criticism over his leadership.

The day after Charles Clarke called for his resignation, the Prime Minister was accused of betraying Labour supporters by the leader of Britain’s biggest union after appearing to rule out a windfall tax on energy companies that had increased their prices despite registering large profits.

Tony Woodley, joint leader of Unite, described the decision as "a downright disgrace", with Hilary Benn, the Environment Secretary, forced to use a lunchtime television interview to deny that the government had "caved in" to big business on the matter.

“If we don’t do that (impose a windfall tax), then we will have betrayed our people and our party,” Mr Woodley said.

“If you don’t turn around and don’t stand up now against vested interests and just work for ordinary people, then don’t be surprised when this country fights back and . . . in the Labour case, sees them go into opposition.”

A Labour backbench MP appeared to make a similar threat last night as he called on the weakened Prime Minister to hit energy firms with a windfall tax and give poorer families a one-off cash gift.

Fabian Hamilton, MP for Leeds North East, said that the consequences for Labour ministers ignoring the plea could be “very serious indeed”.

“They could further lose support from those people that look to the Government for the help that they need right now,” he said.

“And I think that people like me - who are strong supporters of Gordon Brown and the current government as it’s constituted - I think that our support might fade away considerably if we don’t see the very people that we are trying to help supported by Labour.”

Mr Brown arrived in Scotland last night to tell the Scottish Confederation of British Industry that the Government would work with utility firms "to address the problems caused by the impact of world oil prices on gas and electricity bills".

“Not short-term gimmicks or giveaways, but firm steps towards making every home in Britain more energy efficient, thus reducing bills not just temporarily, but permanently,” he said, signalling that he was unlikely to favour a windfall tax.

In his lunchtime interview with the BBC, Mr Benn denied that the Government had buckled in the face of pressure from the energy companies.

“Nobody has caved in to anybody,” he said. “The really important thing is that we get those bills down for the long-term, permanently.”

Today, the Prime Minister will make the short journey to the constituency of Glenrothes, which will stage a by-election in the coming weeks after the death of Labour MP John MacDougall.

No date has been set for the by-election, in which Labour is defending a majority of 10,664. The vote will come just a few months after the shock defeat by the Scottish Nationalist Party in the Glasgow East by-election.

Mr Brown was due to open a trade union learning centre in Glenrothes and visit a voluntary centre in Levenmouth, but Labour insisted it was not a campaign visit and that he would not be accompanied by Lindsay Roy, Labour’s candidate.

A party spokesman said both private visits were in Mr Brown’s role as chancellor of the Adam Smith College in Fife.

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