Mo Mowlam has joined Blair's Labour critics
Writing in the Sunday Mirror she said she now found it "harder and harder to defend what the Labour Government is doing".
The former Northern Ireland secretary frequently topped popularity polls among the public and within her own party.
She adds: "We have a prime minister who has thrown away the British constitution and seems to see himself as our president."
Ms Mowlam's comments come as the longest serving Labour MP, Tam Dalyell, suggested it might be time for there to be a leadership challenge.
And in a separate development the former newspaper editor and high profile Labour supporter Rosie Boycott has quit the party to join the Liberal Democrats.
'Given up Britain for Lent'
Ms Mowlam writes of the PM: "He is embracing international politics with such enthusiasm you would be forgiven that he is fed up with the many domestic problems this country faces.
"As Rory Bremner said last week: 'As it is, it appears the PM has given up Britain for Lent'."
The UK was drifting towards an "offensive, not defensive" war with little debate in parliament or government, she wrote.
"It is not surprising that people are beginning to notice. Forty six per cent of the electorate are now disenchanted with the prime minister - up seven per cent on last week," added Ms Mowlam.
"Even the cabinet has noticed - as one of them said at the Chequers away day 'People think we a load of shysters'."
Rosie Boycott defects
Ms Mowlam also laments the fact that "the almost impossible has happened - New Labour is seen as sleazier than the last Tory government".
Meanwhile Ms Boycott, a founding member of feminist magazine Spare Rib in the 1970s and more recently editor of the Daily Express and Independent on Sunday, said Mr Blair's government was "more reminiscent of a dictatorship than a free healthy democratic system".
Writing in the Mail on Sunday she said New Labour was run by a small number of people who "brook no criticism and turn savagely on anyone".
She adds: "Five years on, those sunny days of May 97 seem more like a dream than a reality and I've given up my Labour Party membership after a lifetime of support."
The question of whether Mr Blair might face a leadership challenge was raised by Mr Dalyell according to comments in the Sunday Times.
'Overstaying welcome'
He said the private calls for a challenge were not coming from the "usual suspects" of those known to oppose the New Labour leadership.
"Anybody after more than five years in Downing Street has to reflect. Margaret Thatcher stayed too long and I think John Major stayed a bit too long in power," said Mr Dalyell.
"I have a general view that, in my 40 years' experience, it is not a good thing for prime ministers to overstay their welcome."
The newspaper says that Labour loyalists scoff at talk of a challenge, saying it is limited to a handful of Labour MPs.
But it comes at a time when 117 Labour MPs have signed a House of Commons motion warning against joining a US attack on Saddam Hussein's Iraq.
Under Labour Party rules a leadership challenge can not be triggered by rebel MPs alone - they need trade union support and backing of constituency parties and a vote at the Labour Party conference.