Gordon Brown is to appeal to the country to give him a second chance as prime minister when he makes a chastened speech to Labour’s party conference.

Early drafts of Brown’s keynote address are said to strike a humble tone, acknowledging the scale of disillusionment with the government’s performance and his role in the party’s slumping fortunes. An insider said: “It will be something of a mea culpa.” However, in a bullish finale, he is expected to insist that Labour can still win the next election, and declare his intention to lead the party to victory.

Brown’s political survival hinges on his performance at the conference, which begins on September 20, with intense pressure for a barnstorming speech. Although no cabinet minister has publicly criticised Brown, many are known privately to agree with the blistering attack on his record delivered by Charles Clarke, the former home secretary, last week.

The scale of disillusionment within the party will be exposed this week as trade union leaders gather for their annual conference. A string of union bosses are expected to declare the government has lost its way. Derek Simpson, the joint general secretary of Unite, which represents 2m workers, said: “It is dawning on more and more back-benchers who are worried about their seats that we have to change the message, and that time is running out. There is a great deal of frustration out there – the government has not shown it is alive to the worries of working people.”

Brown will come under renewed pressure this week to impose a windfall tax on energy companies to help fund payouts to poorer families hit by rising fuel prices.

Yesterday the prime minister’s official spokesman insisted “all options” including a windfall tax remained on the table. However, Treasury sources made it clear that Alistair Darling, the chancellor, is fundamentally opposed to such a levy.

It is expected the government will this week announce a three-year package of energy saving and efficiency measures funded by the utility companies, worth about £1 billion. But the number of Labour MPs demanding a windfall tax has grown to about 130 and they warned that the package would not be enough to halt their campaign.

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