Who is Labour’s backbench stalking horse? UPDATE: It’s categorically not Cruddas UPDATE: Chief Whip names names

By Alex Smith / @alexsmith1982

UPDATE: Labour’s Chief Whip and close Brown aide Nick Brown has tonight told reporters that those behind the email include Stephen Byers, Alan Milburn, Graham Allen, Graham Stringer, Paul Farrelly.

For his own part, Farrelly has told journalists he has nothing to do with the letter and that he is “absolutely furious” at the Nick Brown’s deliberate “smoking out” of backbench dissent.

Having called on members of the cabinet to challenge the PM last summer, Stringer’s involvement is not out of the question, but he would also be an easy scapegoat.

UPDATE: It’s not Cruddas:

A source close to Jon Cruddas has categorically told me that any suggestion that Mr Cruddas is or has been involved in the circulation of the letter calling on Gordon Brown to resign is “complete and utter rubbish”. The source said Mr Cruddas has been “wholly focused” on campaigning and fighting the BNP in his Dagenham constituency, where he has been on the doorstep all day and for much of the last week.

Meanwhile, PoliticsHome has an excerpt of the letter, which reputedly says:

“We are writing now because we believe that in the current political circumstances you can best serve the interests of the Labour Party by stepping down as Prime Minister.”

The snippet was obtained through the BBC’s Nick Robinson.

Speculation is now spreading that Graham Allen, Labour MP for Nottingham North, may be involved in the letter’s circulation.

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With a letter reportedly doing the rounds of backbench Labour MPs, no one has yet been able to confirm who the leader of the rebels is.

Charles Clarke has a history of this sort of activity, but the Guardian reported that many Labour MPs are not signing the letter because it is being circulated by a renowned trouble-maker and championed by those that are “too left wing”.

That leaves two viable liklihoods:

Frank Field and Jon Cruddas.

Qui bono?