David Miliband thrusts himself back into the debate over Labour's future today as he puts forward "my pitch" and issues a coded message to the 58-year-old Prime Minister: " 'Future' is the most important word in politics."

The Foreign Secretary, who remains one of the most likely people to lead Labour after the election, challenged Gordon Brown to use his last conference before the vote to show the "vision" necessary to avert catastrophic defeat.

And on the eve of the conference in Brighton, it emerged that Sue Nye, the closest member of the PM's inner circle beyond his wife, Sarah, is planning an exit from Downing Street amid claims that she is unhappy about the increasing influence of a younger member of the No 10 war room.

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Ms Nye, at Mr Brown's side for more than a decade, is being lined up for a peerage and is in need of "some distance" and a "change of scenery" from the tension of Downing Street. Insiders said the PM's gatekeeper and head of government relations was "really fed up" with the promotion of Kirsty McNeill, 28, from speechwriter to head of external affairs.

Mr Miliband made clear in an interview with The Independent on Sunday that he was positioning himself at the front of the pack in the event of the Prime Minister's resignation. While insisting he was "joining the Gordon Brown stays as PM campaign", his use of the word "future" will be seen as a thinly veiled challenge to Mr Brown to start preparing for a succession.

Mr Miliband's comments – at the start of a week in which Mr Brown will try to position himself to fight off another possible coup attempt – echo his challenge to the PM in the summer of 2008 when he called for "renewal".

At conference this week, the Foreign Secretary said, the party needs to "fight the myth that New Labour doesn't have ideas and that the Government has not got serious and innovative thoughts about the future".

The Prime Minister is under pressure to hand the leadership over to a younger, fresher successor and, in what will be seen as a pointed message to Mr Brown, Mr Miliband said: "I now think 'future' is the most important word in politics." He added that "my pitch" for the future of Labour and the country was to satisfy the "modern aspiration for control".

The Foreign Secretary said that the Brighton conference was an opportunity for Mr Brown to "show what you stand for for the future". He added: "The future is the key, and that means having a mental map that is looking forwards not backwards."

The Chancellor, Alistair Darling, joined in the grim warnings about the future, warning that Labour – from Mr Brown down – has lost "the will to live" and risks handing power to the Tories without a fight.

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Mr Brown reveals today that he will legally oblige all future governments to reduce Britain's debt mountain in an attempt to achieve a sustainable recovery. But, while building in years of severe spending cuts – and potential tax rises – he will also pledge to help the "squeezed middle classes" who, he claims, have suffered most in the recession.

"These are the people who I identify with," the Prime Minister told the Sunday Telegraph. But the latest attempt to revive Labour came as a survey suggested that the party could be permanently out of power. YouGov claimed a haul of only 200 seats at the next election, combined with Tory plans to slash the number of constituencies, and the loss of dozens more Labour seats if Scotland gains independence, could leave the party with no chance of forming a majority again.

Fellow cabinet ministers will use the conference to demand that Labour get ready to "fight" the toughest election campaign against the Tories in 18 years. The Secretary of State for Wales, Peter Hain, writing in the IoS, says Labour is not ready for the fight. Ed Balls, the Secretary of State for Children, said: "What we need is more fighters, not quitters."

Ms Nye's name is said to be on a provisional list of Labour peers, which has been delayed for months, allowing her departure from No 10. A Downing Street spokesman said: "It's not true that she is leaving. She is going to be planning the election campaign. She is not thinking of an exit, and has no active plans for a peerage."

However, after the story broke last year that Stephen Carter, then head of strategy at No 10, was planning to leave and was in line for a peerage, Downing Street issued a denial. Yet a month later, Carter was moved to a ministerial post in the House of Lords.

Meanwhile, the profitable post-Downing Street life of Tony Blair goes on. According to a report in The Sunday Times, devotees and admirers who attend his lecture tour events are paying £180 a pop to have their photographs taken with the Great Man.