Labour will have to "defy political gravity" and renew itself, if it is to win the next general election, says Environment Secretary David Miliband.
Mr Miliband is a close ally of Tony Blair's
Governments which won a fourth term were rare, as voters got bored with them, he told the Daily Telegraph.
To win, Labour needed "new leadership, new strategy and new culture".
Mr Blair agreed with Mr Miliband's analysis, while giving a clear hint that he would not stand down as prime minister before the end of June.
He has already announced that he intends to resign by the start of September, and some have speculated he might go after the Scottish, Welsh and local elections on 3 May.
But asked on Tuesday whether he would be at a summit of European Union heads of government in Brussels, on 21 to 22 June, he replied: "Of course."
He admitted that governments that have been in power some time would always go through a period when "all the emphasis is on the negative".
But he said Labour had to hold its nerve on "the reform and change programme" to come through it.
He added: "If the worst that you have to put up with is boredom in a mid-term, you are probably doing reasonably well."
Mr Miliband, who is seen by some as a potential future Labour leader, has backed Chancellor Gordon Brown as the next prime minister.
'Boredom and fatigue'
He said: "After 10 years in power, Labour needs to defy political gravity to win the next general election.
"To win an unprecedented fourth term, Labour needs new leadership, new strategy and new culture.
"The first is taken care of by Gordon Brown's qualifications for the premiership.
"The second and third, and the boldness with which we change new Labour to revive new Labour, are the core of our process of renewal."
He told BBC News 24 that the exact timing of a change of prime minister was not important, because, he said, there was a good prime minister and good "prime minister-in-waiting".