Mr Ashdown said he had been offered the job by UN Secretary-General Ban Ki Moon and that Afghan President Hamid Karzai had initially supported the appointment.

But there were increasing signs that Kabul had cooled on the move in recent days, with some reports suggesting that Karzai thought Mr Ashdown, the international community's former envoy to Bosnia, wanted too much power.

'This job can only be done successfully on the basis of a consensus within the international community and the clear support of the government of Afghanistan,' Mr Ashdown said in a statement, after writing to Mr Ban on Saturday.

'It is clear to me that, in Afghanistan at least, the support necessary to do the job effectively does not exist. I have therefore reluctantly decided to withdraw my name from consideration for this position.'

Afghanistan's Foreign Minister Rangeen Dadfar Spanta told reporters on Sunday he wished Mr Ashdown 'success", adding Kabul's objections were not down to Ashdown or his nationality but to a 'negative atmosphere' created around the envoy role.

'It's better if our friends let us learn more and more by walking on our own feet, with our own experience,' he added.

The minister praised Mr Ashdown, saying he was one of the 'very small number of diplomats who has a good understanding of the region.'

The Afghan government had however had concerns about the powers of the job, which had previously been that of a special representative of the United Nations but which international circles had wanted to expand.

The United States embassy in Kabul said it was 'disappointed' at the news.

Ex-Liberal Democrats leader
Mr Ashdown, former leader of Britain's smaller opposition party the Liberal Democrats and an ex-marine, gained a tough reputation during his time in post-war Bosnia from 2002 to 2006.

He pushed through a string of sensitive reforms, which included efforts to merge two ethnically divided armies, and the sacking of 60 officials suspected of belonging to a support network for war crimes suspects.

He is now a member of the unelected upper parliamentary chamber the House of Lords and until recent days had looked all but certain to take up the 'super envoy' role coordinating the international community's work in Afghanistan.

Tensions between London and Kabul
The announcement comes against a backdrop of recent tensions between London and Kabul.

At last week's World Economic Forum in Davos, Switzerland, Mr Karzai was reported to have blamed British and US troops for contributing to worsening security in southern Afghanistan that had allowed the Taleban to return.

His comments provoked a furious response from the families of some of the 87 British troops who have died in Afghanistan since 2001 and a flat denial from Prime Minister Gordon Brown's official spokesman.

But Mr Ashdown said he thought recent tensions between the two countries related to a disagreement over his candidacy rather than wider issues.

Mr Karzai's comments in Davos stirred up anti-British feeling which was 'much more about sending a message to me than it was about sending a message to Great Britain,' he told BBC television.

Mr Ashdown added that he now hoped relations between the two countries would stabilise.

'One of the reasons why I've withdrawn is precisely because I think it's important that Britain's relation with Afghanistan should get back on to an even keel,' he said.

'When you generate these kinds of feelings in a country like Afghanistan, that can cost lives and I wouldn't want to be the instrument of that.'

He also denied ever seeking the same powers in Afghanistan as he had in Bosnia after the 1990s Balkan wars, where some critics compared his role to that of a 'colonial governor'.

'The government of Afghanistan is a sovereign government, it's a proud nation, President Karzai is its president,' he said, adding Karzai may have withdrawn his support for him because of 'internal Afghan politics'.

Zahir Tanin, Afghanistan's ambassador to the United Nations, told the BBC on Saturday that Kabul's preferred choice was Nato's deputy commander in Europe, General John McColl.

General McColl, another Briton, was the first head of the International Security Assistance Force in Afghanistan in 2002 after the hardline Taleban were ousted. -- AFP