Former transport minister Stephen Byers has warned that the coming year will be "make or break time for New Labour".
Stephen Byers says this is a crucial time for New Labour
Writing in the Times, Mr Byers said the party's Comprehensive Spending Review will prompt a "fundamental" rethink of Labour's priorities.
He called for new policies to "improve social mobility" and for security to be given "greater priority".
He wrote: "This is the moment for an open and honest debate about the future direction of the Labour Party."
Mr Byers added that it was important for government to "create public services that match quality and standards in schools and healthcare with the record levels of investment in these areas".
BBC political correspondent Robin Brant said Mr Byers was thought to be speaking with the approval of Downing Street.
"In difficult times this mission statement from Stephen Byers will serve as a rallying cry to Tony Blair's supporters," he said.
"But it's also evidence that even the most ardent loyalists believes it's now months and not years before Tony Blair's premiership comes to an end."
Mr Byers' comments come after a weekend in which a number of senior Labour figures have spoken about the future of the party and of Mr Blair's leadership.
Europe minister Geoff Hoon said voters were starting to see Conservative leader David Cameron as a serious alternative to Tony Blair.
"There are a whole range of issues that lead people to think that, perhaps after a long period in government, there might be an alternative," said Mr Hoon.
"There is no doubt that the Conservative Party looks very different than the one, say, Michael Howard led."
And Culture Secretary Tessa Jowell said Tony Blair's plans to step down as leader were creating "uncertainty" within the Labour party.
"There is undoubtedly a sense of uncertainty and a sense of focus on this issue of the leadership," she said.
"This is very much fed within the Westminster village but it is by no means the overriding issue in the Labour Party."
But the Lord Chancellor, Lord Falconer, said Mr Blair was still the right person to be leading the party.
"This is not the time to change a leader, this is the time to be talking about how we're going to improve schools, how we're going to improve the health service," he said.
Cabinet office minister Jim Murphy suggested the prime minister would not set out his timetable for departure at the forthcoming party conference.
"He's already been more open than any previous prime minister about his intentions," Mr Murphy told the BBC Radio 4 programme Today.
"I think [Mr Byers] is right in saying that we need a new set of ideas and set of policies to take New Labour through to its second decade."