By Bridget Kendall BBC diplomatic correspondent

This letter is highly unusual. It is rare, if not unprecedented, for so many former senior British diplomats to band together for the express purpose of criticising current policy so publicly.

Diplomats admit they see little chance of changing Mr Blair's mind

Even in retirement, ex diplomats - with some exceptions - tend to be diplomatic. But the language they use here is downright scathing.

Ariel Sharon's recent policy shift on West Bank settlements and on the Palestinian right of return, endorsed by President Bush (and in their view by Tony Blair as well) they describe as "one-sided" and "illegal" and "an abandonment of principle".

On Iraq they bluntly assert "there was no effective plan for the post-Saddam settlement", and describe coalition pronouncements as variously "not well judged", "neither convincing nor helpful", "na�ve", and "a disgrace".

Loyal ally

And "doomed to failure" is their overall judgement on the policies they criticise, unless Mr Blair can show he really can exert real influence on the US as a loyal ally.

What is more it appears they expressly chose to make the letter public, instead of sending it to the Prime Minister privately.

But, it seems, public impact is precisely the point of this letter.

The language they use here is downright scathing Bridget Kendall Diplomats slam Blair on Mid-East Ambassadors' letter to Blair

Its aim is to put on the record a widespread sense of unease which those who instigated the letter believe they reflect.

"It was important to do this, if only for the historical record," said one signatory.

Another reflected that among serving British officials, there would probably be a good few who privately shared many of their reservations.

Beyond that, there is a hope that such trenchant criticism will stimulate further public debate about British policy on the Middle East - and towards the United States - both in the media and in the British parliament.

Immediate impact

Few seem to think it will have a more immediate impact on policy.

As for Tony Blair himself, some of those involved in the letter admit they see little chance of it prompting the prime minister to change his mind.

A spokesman said the Prime Minister would reply in due course

One signatory described him as a "true believer", who was unlikely to change his view of policy on Iraq or on the importance of his close co-operation with the current US administration.

The response so far from Downing Street seems to endorse that.

A spokesman said the prime minister would reply in due course, but rejected the idea of a scorecard to keep tabs on influence exerted on the US president.

Inevitably the ex-diplomats have already been dismissed as "Foreign Office Arabists", in retirement, and well known for particular views of British Middle East policy.

But that is not entirely accurate.

There are former British Ambassadors to Moscow, the United Nations and Brussels among the signatories, who do not usually speak up on Middle East policy.