Tony Blair is "losing his grip" and should quit immediately and hand over power to Gordon Brown, according to former Chancellor Lord Healey.
Lord Healey said that Blair had been in the job "too long" in 2004
The Labour elder statesmen has been a long-term critic of Mr Blair's decision to go to war in Iraq.
He told the BBC: "I think Tony's showing he is losing his grip, and the sooner Gordon takes over the better."
His comments came as a poll suggests the public prefer Mr Brown to David Cameron as a future prime minister.
'Time is now'
In an interview on the BBC's Sunday AM programme Lord Healey said the imposition of university tuition fees and the establishment of foundation hospitals were fundamental errors made by the prime minister.
Asked when he thought Mr Blair should step down, he responded: "I think it is time now."
The Labour grandee also called for the prime minister to resign in 2004 because he believed he had been in the job "too long" and his ideas had become "too fixed".
Mr Blair has given no dates for when he would step down
Mr Blair has said he will leave Downing Street before the next election, but has refused to name a date and is signalling he could stay for years yet.
He has strongly endorsed Mr Brown to succeed him as Labour leader, but said he wants to see his election manifesto put into action before he steps down.
A poll in the News of the World puts Mr Brown 12 points ahead of Mr Cameron as voters' choice for next prime minister.
Some 45% of those taking part chose the chancellor as the best person to lead the country, against 33% for his probable opponent at the general election expected in 2009 or 2010.
Respondents voted Mr Cameron as more presentable than Mr Brown - by an overwhelming 61% to 22% - but rated Mr Brown as most in touch with the issues that matter.
For the News of the World poll ICM Research interviewed 1,025 adults between February 23 and 25..