Analysis By Nick Assinder Political correspondent, BBC News website

The fact that left-winger John McDonnell is prepared to enter a Labour leadership contest may genuinely worry Gordon Brown.

Brown might welcome a left challenge

Not, however, because the chancellor fears defeat at the hands of the rebellious backbencher.

The real worry is over whether the mere fact that there is a contest might persuade other more likely candidates to throw their hats into the ring.

There has already been talk of a rival Blairite candidate such as Home Secretary John Reid or former minister Alan Milburn pitching in to keep the flag flying when the time comes.

Similarly, there has been speculation that some believe the gloss has already started coming off Mr Brown when compared to David Cameron, and may look towards a younger, next generation candidate.

The name David Miliband keeps getting mentioned - not least by Tony Blair himself, albeit not as a candidate but simply a shining future hope.

And education secretary Alan Johnson's name is also mentioned as a possible future leader although others see him as a more likely deputy at this time.

Iraq war

Mr Brown would almost certainly welcome a challenge from the left which he will feel confident he can see off.

It would not only underpin the message that he is not one of those, Old Labour lefties - if any still suspected it - but it would be seen as legitimizing his leadership.

McDonnell leads Socialist Campaign Group of MPs

There had been increased speculation that the left would field a candidate after Mr Brown infuriated them by announcing his support for the Trident nuclear deterrent.

Former minister Michel Meacher was mentioned as a possible candidate, but some on the left had reservations about him because he did not resign his job over the Iraq war.

Some have even claimed Mr Brown would welcome the chance to see off a Blairite candidate although it is unlikely he would want the sort of divisive contest that might spark.

Similarly, it still seem most likely that, if and when the time comes, it will be a straight fight between Mr Brown and the left and others will hold back for fear of splitting the party at what is already a hugely difficult time.

But the fact that the left is so determined to challenge the chancellor is the clearest indication that, should he take over from Mr Blair, he is likely to face exactly the same sorts of rebellions the prime minister long ago learnt to live with.