ALAN MILBURN was urged to "put up or shut up" last night after launching what was immediately dubbed a Stop Gordon Brown campaign at Westminster.

A senior Labour backbencher urged the Darlington MP to "put his hat in the ring" if he believed there should be a contest to succeed Tony Blair as Prime Minister.

Tony Wright, the chairman of the public administration select committee, spoke out after Mr Milburn and fellow ex-Cabinet minister Charles Clarke unveiled a website to stimulate an "open debate" on Labour's future.

The pair were forced to deny their aim was to stop Mr Brown reaching No 10 by smoking out a heavyweight challenger - perhaps David Miliband, the Environment Minister.

Mr Milburn and Mr Clarke fuelled further suspicion when they failed to rule themselves out as candidates when Mr Blair steps down, possibly as soon as May.

And Mr Clarke admitted that Mr Brown, although welcoming the "debate" when they discussed the move earlier this week, had raised concerns about the timing and conduct.

Asked about the launch, Mr Wright said: "If people think we need a contest for the leadership, then they should put their hats in the ring.

"Both Charles Clarke and Alan Milburn are very credible candidates. If what they really want is a contest, they should have one. They have it in their power to have one."

Denis MacShane, the former Europe Minister, added to the criticism, warning that Labour was in danger of turning into a debating society instead of being an effective government.

At the launch at a Westminster hotel, Mr Milburn, the former Health Secretary, pointedly said the debate on Labour's future policies must be a "dialogue, not a monologue".

But, speaking later, he insisted he was launching the website - www.the2020vision.org.uk - to influence what the Labour Party stood for, not who led it.

Existing websites inviting people to comment on party policies - such as Labour's Let's Talk initiative - did not do enough to involve the wider public.

Mr Milburn said: "If parties don't talk to the public, they find very quickly that the public has stopped listening. If parties don't talk about the future, they don't have one."

He welcomed Mr Brown's recent wider-policy speeches, but added: "I think that should be actively encouraged and it shouldn't be limited to just one individual."

Asked whether he would vote for the Chancellor, Mr Milburn said: "I will be voting for whoever I decide is the best candidate when we know who all the candidates are. That is something for a future date."

The launch came amid growing Labour nervousness after polls suggested Tory leader David Cameron is far more popular than the Chancellor, long thought the Prime Minister-in-waiting.

So far, only two contenders from the left of the party - John McDonnell and Michael Meacher - have said they intend to challenge Mr Brown.

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