Tony Blair's administration has been dealt what may turn out to be a fatal blow by the resignation of seven members of his Government. 

Tom Watson, the under-secretary of state for defence, wrote to Mr Blair this morning telling him that it was no longer in the interests of Labour or of the country for him to remain Prime Minister.

This afternoon six parliamentary private secretaries - MPs on the lowest rung of the Government ladder - also handed in their resignations.

Khalid Mahmood, Wayne David, Ian Lucas, Mark Tami and David Wright said they had to leave because Mr Blair had not "ended the uncertainty" over when he intends to step down.

Chris Mole also quit his post as parliamentary private secretary to Phil Woolas, communities minister.

The Prime Minister responded immediately by describing Mr Watson as "disloyal, discourteous and wrong" and saying that he had planned to fire him anyway.

And he warned Labour MPs that they risk consigning the party to electoral defeat if they continue to call for his resignation.

The six Government members who resigned today are all understood to be among the 17 MPs who signed a confidential letter to Mr Blair yesterday urging him to name a date for his departure.

Mr Watson's resignation will have all the more impact as he is known as a Blairite loyalist, who formerly served as a Government whip and was expected to rise through the ministerial ranks under the Prime Minister.

In his letter, released to the press, Mr Watson wrote: "It is with the greatest sadness that I have to say that I no longer believe that your remaining in office is in the interest of either the party or the country.

"How and why this situation has arisen no longer matters. I share the view of the overwhelming majority of the party and the country that the only way the party and the Government can renew itself in office is urgently to renew its leadership."

Last night it was reported that Mr Blair had set a date for his departure of May 31 next year, in an effort to curb demands for his immediate resignation.

An eight-week leadership election campaign will follow allowing a new leader — the favourite being Gordon Brown — to be chosen by July 26, when Mr Blair will step down as Prime Minister.

No 10 described the claims as "speculative" but refused to deny their accuracy.

If true, it would validate a report in The Daily Telegraph at the end of June that Mr Blair had decided to go next May after 10 years as Prime Minister.

Yesterday two Blairite Cabinet ministers, David Miliband, the Environment Secretary, and Hilary Armstrong, the Chancellor of the Duchy of Lancaster, said they expected Mr Blair to go within a year.