Analysis By Nick Assinder BBC News Online political correspondent

Mr Mackinlay pointed to trouble ahead

Tony Blair should ring up President Bush and ask him to withdraw the request because Labour MPs would never approve it.

Mr Mackinlay has characteristically put his finger on the nub of the prime minister's dilemma over the US request.

First, his suggestion recognises that the only way British troops will escape being sent to help out the Americans is if the president withdraws the request.

But, probably more importantly for the prime minister, it also acknowledges what many believe to be a real threat to Mr Blair - a major backbench rebellion which spreads far beyond the "usual suspects".

Of course, it is highly unlikely the prime minister will decide this decision should go to the Commons for a vote. And, under normal circumstances, that would not be necessary or demanded.

Last straw

But under current circumstances, to press ahead without the explicit backing of the Commons - or a full debate at the very least - could serve to turn normally supportive MPs into opponents.

Because, for many loyal backbenchers who supported the decision to go to war on Iraq, this is the last straw.

Labour MPs who voted for the war and who are now expressing severe doubts over the US request include such senior figures as Sir Gerald Kaufman and once-supportive backbenchers including Eric Illsley, Geraldine Smith and a host of others.

They have joined the "usual suspects" like Alan Simpson, Alice Mahon and Glenda Jackson in expressing serous concerns over the latest developments.

Just as they were hoping for signs of an exit strategy from Iraq, they are presented with what Liberal Democrat leader Charles Kennedy has branded an "ensnarement strategy" instead.

And it was pretty plain from the contributions to Defence Secretary Geoff Hoon's difficult Commons statement on the issue on Monday that they are ready to defy the prime minister over this "step too far".

But what has made these otherwise supportive MPs turn on Mr Blair is not only the fear of being drawn deeper into the conflict, serious though those concerns are.

British troops may back-up US forces

It is also that many of them have come to the end of their patience with the relationship between their prime minister and the Republican president.

That relationship has always been a cause of concern on the Labour benches. The fear now is that Mr Blair is either conniving with Mr Bush in the run up to the US election or is, at least, being used by him.

Straw smear

And feelings are running high over this latest development.

The clearest sign of that was the unusual move by Mr Kennedy to ring up the BBC's Today programme demanding an apology from Foreign Secretary Jack Straw after he had been interviewed about the issue

Mr Straw had suggested that, had the opponents of the war had had their way, nothing would have been done to tackle Saddam Hussein who would, as a result, have been strengthened.

Mr Kennedy was infuriated by what he regards as a smear and is to press ahead with his demand for an apology.

But the suggestion will also anger many Labour backbenchers who believe there were other ways of dealing with Saddam, including a continuation of the policy of containment.

So Mr Straw's words are likely only to escalate this latest row rather than damp it down.

And denying MPs a way of expressing their opposition to the planned troop re-deployment is unlikely to help either.

As Mr Blair has previously learned, rebellious MPs find plenty of other ways of expressing their anger- notably through votes on other pieces of government legislation.