Although news that Mr Blair's wife, Cherie, was pregnant overshadowed the past few days' headlines on Labour's selection contest, the prime minister found time to author an article for the London Evening Standard newspaper.

In it he said Mr Livingstone's time at the Greater London Council had coincided with the party's "lowest ebb" in the capital.

Mr Blair also said that unless Mr Livingstone - who on Thursday eventually made it onto the party's shortlist of candidates - had given up the "extremism" of the 1980s, he would not make a good mayor.

"Now, this is the issue: has Ken Livingstone really changed? If he hasn't, he would not be right for Labour or London," Mr Blair writes.

"Neil Kinnock, John Smith, me and the members of the Labour Party did not go through all the struggle and effort we did to create New Labour only to throw it all away and return to the disastrous politics of the early '80s."

Blamed for wilderness years

He said that he accepted Londoners had not wanted to see the GLC abolished by then prime minister Margaret Thatcher in 1987, but insisted this did not mean Labour was popular in the capital at that time.

He said that in the 1987 general election London was the only region in Britain to record a swing to the Conservatives.

Mr Blair's attack made clear that the main thrust of the party hierarchy's attack on Mr Livingstone was not simply to identify the popular left-winger with Labour's wilderness years during the 1980s, but to attempt to hold him personally responsible for them.

"At that time the Labour Party was a byword for extremism. We were hopelessly divided and deeply unpopular. MPs and council leaders routinely attacked the leadership.

"The atmosphere in meetings was poisonous. We had an agenda that maybe appealed to our own activists but it was a million miles from what the public wanted.

"The leading figures in the Labour Party were people like Ken Livingstone, Tony Benn and Arthur Scargill. The policies were not just disastrous for Labour. They deprived the public of a choice that wasn't the Tories."

Second attack in two days

The prime minister's official spokesman confirmed that Mr Blair planned to take a prominent role in the mayoral contest.

He also insisted that Mr Blair's opposition to the Brent East MP's bid to win the Labour nomination was not a personal attack on Mr Livingstone, but an argument about the modern Labour Party.

Mr Blair's strong criticism of Mr Livingstone on Friday was his second attack in as many days. The first came just hours after the party decided he would be allowed to go forward to the next stage of the selection contest.

Deciding ballot

Now, a three-way electoral college - one section party members in London, another of MPs, MEPs and Greater London Assembly candidates and the third of affiliated trade unions in the capital - will decide the winner, though not until next year.

The party, which has repeatedly postponed the selection process, has once again put the date back. The deciding ballot will now take place in a three week period between 26 January and 16 February.