The Prime Minister faced an angry backlash after he used a keynote speech on Thursday night to make it clear there would be no "short-term gimmicks or giveaways" when the Government unveiled its much-vaunted energy plan next week.

Trade unions and Labour MPs responded by launching fresh demands for a windfall tax on the profits of the energy companies.

Environment Secretary Hilary Benn rejected claims that the Government had buckled in the face of pressure from the energy companies.

"Nobody has caved in to anybody," he said.

He said that discussions with the companies were continuing and that the Government still expected them to make a "contribution". He refused to be drawn on whether a windfall tax had been ruled out.

"I think it is only fair that the energy companies should make a further contribution to assist customers this winter," he told BBC Radio 4's The World at One.

"The discussions that are taking place are about what is the best form that that help should take."

General secretary Tony Woodley of Britain's biggest union, Unite, warned that voters would punish the Government unless it showed that it was prepared to stand up for "the ordinary people in our country".

Mr Woodley said: "Our party will say to this Prime Minister...if you don't turn round now and stand up against vested interests and work for ordinary people, then don't be surprised when this country fights back and in the Labour case sees them go into opposition or in the Tory case doesn't even see them go into power."

Labour backbench MP Fabian Hamilton said that Mr Brown's Premiership could be under threat unless he took action.

"I support Gordon Brown and I want to see Gordon as our Prime Minister," he told BBC News.

"But the fact is he will only continue as our Prime Minister and we will only continue in government if we can show people the reason for having a Labour government is to help the poorest, the most vulnerable, the weakest, those who are unable to pay these bills through no fault of their own."

The issuing of fuel vouchers worth between £50 and £100 to up to seven million of the poorest households had been expected to form the centrepiece of next week's energy package.

Mr Brown, however, made it clear that the announcement next week would concentrate instead on measures to improve energy efficiency.

But Age Concern director general Gordon Lishman said that people urgently needed help with their payments this winter.

"Pensioners up and down the country will be devastated if extra help for those struggling to afford their bills is rejected. This would usher in a winter of discontent for the one in three pensioners likely to be in fuel poverty," he said.