Ministers are holding the threat of a windfall tax over energy companies to win guarantees that the costs of new moves to cut household bills will not be passed on to consumers, The Times has learnt.

Gordon Brown will face calls for a one-off tax on the profits of the big companies when he meets union bosses behind closed doors at the TUC conference in Brighton tomorrow.

The Prime Minister and Alistair Darling, the Chancellor, remain reluctant to impose such a tax. However, in light of the failure last week of his attempt to raise more money from the companies to fund fuel vouchers for the vulnerable worth up to £100, Mr Brown wants to ensure the cost of a new scheme to help families to make their homes energy efficient, permanently reducting their bills, is not pushed back on to the consumer.

Senior ministerial sources told The Times that, if energy companies tried to pass on the cost, the windfall tax idea would be resurrected swiftly.

Mr Brown wants energy companies to pledge £300 million in each of the next three years to help the most needy households to modernise boilers and insulate their homes. He also wants companies to deal with the problems of prepayment meters, which cost poorer customers hundreds of pounds more a year.

Mr Brown's dinner with the TUC General Council comes at the start of a four-week conference season that will determine whether his relaunch has succeeded.

As The Times reported ten days ago ministers and backbenchers who want Mr Brown to be ousted or to stand down have backed away from any immediate move against him. The Glenrothes by-election, expected late next October or November, has become the next possible “tipping point” as far as the dissidents are concerned.

Although Charles Clarke was the only senior figure to speak out publicly last week, he is far from alone in his grim analysis of Labour's plight. Many Labour MPs and ministers have doubts over whether the party can recover under Mr Brown. That is why the Prime Minister is putting so much into his speech at the Labour conference in two weeks.

His foreword for a Cabinet report to the conference is said to contain much of the flavour of the speech which, as his closest ally Ed Balls, the Schools Secretary, said yesterday, will have to show the party it can still win.

Mr Brown will use it to show his critics that he does have a vision to take Labour into another phase of government. In the foreword that will be distributed to members of the Cabinet today he promises support for people struggling with the impact of global insecurity, investment in skills, a new deal for the elderly, help for parents and decisions on transport, energy and climate change.

His words are clearly an attempt to reassert his authority but they are also an assurance to the Cabinet that he will work with them to help to find the answers to the new challenges facing the country. They also have a more personal touch, which Mr Brown is expected to exhibit in his conference speech. Close aides wish the country could see more of Mr Brown as the family man, arguing with him that David Cameron shows no inhibitions in displaying his family to the public.

Mr Brown remains cautious on that front but a statement in the foreword that he has overcome personal challenges before, and will do so again, was a reminder to his opponents that he will not go down without a fight.

The Prime Minister compares Labour's challenge today to that which faced the party in 1945 and 1997. He goes on: “Our task, as a party, as a government, now is to ensure that our country and all of our people make the most of these opportunities and are protected against the risks that accompany radical change.

“The quest to ensure that power, wealth and opportunity are in the hands of the many, not the few, is fundamental to our purpose. This is why the Labour Party exists. It is what enabled us to rise to the challenge of the times in 1945. And again in 1997. And it is why we will now rise again to meet the challenges of this new world.”

Mr Brown sees Labour's challenges as being:

— Economic restructuring; Labour needs to ensure that everyone has the skills to work and prosper in this new world;

— Growth intensifying the pressures of climate change;

— Greater freedoms bringing new strains on communities and family life; the boundaries which mark out civilised society have been eroded; new risks from abroad mean the country needs to reduce its dependence on oil to ensure energy security; new threats from terror need to be tackled at source and require a more joined-up response;

— Parents facing new pressures of time and money in raising their family and the increasingly difficult task of juggling work and family life;

— People live longer and healthy lives but Labour needs to ensure that everyone in the older generation who has given so much to the country receives the security in old age they are entitled to expect.

TUC conference schedule

Monday

Speeches by Brendan Barber, TUC General Secretary, Dave Prentis, Unison general secretary, who is this year’s TUC president.

Debates: public sector pay, pensions, equality, employment rights. Flashpoint is trade union legislation

Tuesday

Speeches from John Denham, Universities Secretary, Alistair Darling, the Chancellor.

Debates: the economy, with unions pressing for windfall tax on power companies; public services, learning and skills, protection of vulnerable workers.

Gordon Brown meets union leaders later at private dinner

Wednesday

Speeches from Harriet Harman, Labour deputy leader, Tarsicio Mora Godoy, president of Colombian trade union centre.

Debates: equality, union organising, transport and international affairs, including European legislation on Europe’s social model

Thursday

Debates: Employment rights and health and safety debates

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