Tony Blair will declare that he intends to serve a full third term as Prime Minister if Labour wins the next general election in a move that would mean he remains in Downing Street for another five or six years.
Mr Blair's message, which will risk comparisons with Margaret Thatcher's pledge to "go on and on", will infuriate his Labour critics at a time when his leadership is under renewed fire because of the deepening problems in Iraq.
It also risks further widening the split between Mr Blair and Gordon Brown, who is furious at being sidelined from his key role in Labour's election campaign. That was handed to the Blairite Alan Milburn in this month's cabinet reshuffle.
Blair aides told The Independent yesterday that a decision had been taken in principle that Mr Blair would state his intention to serve a full term before the election. One option was to "clear the air" by saying it before Labour's annual conference in Brighton, which starts tomorrow. But allies say this will not happen now as it would be insensitive to talk about his long-term ambitions during the hostage crisis in Iraq.
After coming through his "wobble" this spring about whether to stand down this year, Mr Blair seems determined to stay on for longer than expected. There is a widespread view at Westminster that he will leave No 10 in 2006 after a referendum on the European Union constitution.
Close aides insist that Mr Blair's move is not a slapdown to Mr Brown, seen by many in the Labour Party as the clear front-runner to succeed him. They insist that the Prime Minister wants the Chancellor to be a "big player" in a "team leadership". Mr Brown's supporters fear that, the longer Mr Blair stays on, the more the Chancellor's chances of becoming Prime Minister will recede. At 53, he is two years older than Mr Blair and some Blairites believe he has "missed his moment". They say Mr Blair will risk a backlash from the Brownites by serving a third term because the Chancellor proved he was unwilling to be "an assassin" this year.
Mr Blair's pledge will also risk accusations of arrogance. His aides insist that he does not want to talk about his future but he has to come up with a formula to answer the questions about how long he intends to serve. The Conservatives have already floated plans to use "Vote Blair, Get Brown" as an election slogan.
A close Blair ally said yesterday: "We don't want the election dominated by speculation about whether Tony will serve for another one, two or three years. So he will say that he stands for election for a full term.
"It's not aimed at Gordon [Brown]. He will probably go ballistic but he never plunges in the dagger."
Although the Labour conference may be eclipsed by events in Iraq, it may provide a clue to Mr Blair's long-term intentions. Aides say he is "re-energised" about the Government's domestic agenda and determined to see through the five-year plans on health, education, transport and crime unveiled this summer. "The aim at the conference is to show that we fight for a third term on a New Labour agenda and that it will be a Blair-led government that implements it," one cabinet minister said.
Mr Blair has always been anxious not to repeat Baroness Thatcher's mistake of overstaying her welcome in Downing Street. In 1987, she pledged to "go on and on" and won her third election. She rejected the idea of quitting after 10 years as Prime Minister in 1989 and the following year was forced to resign by her Cabinet and MPs.
If he completes an election hat-trick, Mr Blair would complete 10 years at Number 10 in 2007, an obvious time to reconsider his own "on and on" pledge. But colleagues believe his new-found confidence and decision to reassert his authority over the Cabinet mean that he could stay on for longer than 10 years and lead Labour into a fourth election.