Gordon Brown was advised by a key Labour strategist to hold an early election after his "significant honeymoon" as prime minister.
Gordon Brown was warned the next election will be 'tough'
Labour pollster Philip Gould told Mr Brown, before he became leader, that he needed to be "a powerful, muscular modernisation politician".
His advice, leaked to the Daily Mirror, included having "a short period of intense and compelling activity".
A Whitehall source told the BBC that the memo was written two years ago.
Cabinet Minister Ed Miliband, in charge of writing the Labour manifesto, also said he could not remember seeing the document.
It was apparently written just after the 2005 general election when discussions were taking place about a transition from Tony Blair to Mr Brown.
'Shock and awe'
In the leaked memo, published in edited form on Thursday, Lord Gould warned that the next election will be much tougher than the last for Labour.
To hold on to - and improve - its majority, Labour needed "a bold election strategy designed to dramatically drive our vote up".
He added: "We have to have a strategy of audacious advance. The best way of achieving this is to hold an early election after a short period of intense and compelling activity.
"A kind of 'shock and awe strategy' blasting through the opposition."
Lord Gould was an influential adviser to Mr Blair and seen as a key New Labour architect.
Confident that Mr Brown would take over from Mr Blair, Lord Gould also said: "It is inconceivable that you will not enjoy a significant honeymoon when you become leader.
"You need to build on this and translate it into a new mandate. I am sure this strategy will work."
And he adds: "You must start election planning early. We can't leave it late as we did last time. We must make a start."
The Whitehall source told the BBC not to "rule anything out" regarding the timing of a general election but suggested that it was more likely to be next year than this autumn.
Lord Gould urged the then chancellor, who became leader in late June, to "exemplify renewal, change, and a fresh start. Your premiership has to have a dynamism and an energy that pulls people along in its slipstream. You must become the change Britain needs."
Trying to emulate the "charismatic" Mr Blair would be "foolish", Lord Gould warned, but he was optimistic that Mr Brown would be able to present his own "true potential as a person and a leader".
"It will not be enough just to be different to Tony. You should be identifying challenges and driving the nation forward to meeting them. You should own the future.
"When you become leader, you must unleash your power and energy and emerge as the compelling politician and person that you are.
"Your own distinctive charisma will then emerge."
The UK prime minister can call an election at any time up to May 2010. Recent practice has seen an election every four years, which is why May 2009 is still seen as the most likely date.
Mr Miliband refused to be drawn on the timing of the next election, saying it "needs to be ready whenever is necessary".
"We've been in for five weeks or so. It's important not to get carried away by opinion polls," he said on BBC Radio 4's World at One.
"We've got a big job to do to earn people's trust and show that on the issues that matter - housing, health and education - we can make the changes that people really want to see."
He emphasised that the decision on an election was up to Mr Brown.
Mr Miliband said Mr Brown had shown "strength and steadiness in the face of a crisis" as in his first days and weeks as leader there were attempted terror attacks in London and Glasgow and then severe flooding in parts of England.
Former Labour Party treasurer Lady Prosser said the party did not have the money to campaign at the moment - but also said the speculation was not a ploy to shore up support from donors.
"I would have thought there would be greater reasons for donors to want to contribute to the Labour Party rather than simply for a snap election - I think the new prime minister and a sort of new start for Labour.
"I don't doubt that there are quite a few people out there going to be prepared to give, but that's not just come through just quite yet."
Both Liberal Democrat leader Sir Menzies Campbell and Conservative leader David Cameron have said they would welcome an election as soon as possible, saying their parties were ready for any snap poll.