Paddy Ashdown was said today to have agreed terms for him to become the new United Nations “super envoy” to Afghanistan.

Lord Ashdown of Norton-sub-Hamdon, the former Liberal Democrat leader, was said by United Nations sources to have accepted the role. A formal announcement is expected to follow, although his precise responsibilities have yet to be confirmed.

He has been keep to do the job but has insisted on been given enhanced powers to give him the scope and authority he felt were needed to undertake the role. Otherwise he felt the job would not be worth doing, he had told friends.

His conditions required extra negotiation and approval, particularly from the Americans. There were also sensitivities within the Afghan government about beefing up the role in a way that carries echoes of a British colonial-style governor.

Lord Ashdown, who left the Commons in 2001, two years after stepping down as leader of the Liberal Democrats, is a former Royal Marine Commando and special forces officer.

He was held to have acquitted himself well in his previous role as the international administrator, as the European Union’s special representative in Bosnia-Herzegovina from 2002 to 2006.

He will replace a German diplomat, Tom Koenigs, who has had a low profile as the UN’s Special Representative in Afghanistan.

As part of his wish for a stronger mandate, Lord Ashdown has sought to combine the post with some of the former responsibilities of Nato’s civilian envoy to Afghanistan, Daan Everts, who concluded his mission at the end of last year.

Mr Koenigs had expressed his frustrations at the lack of coordination between the various missions and international agencies in Afghanistan, where Nato has 42,000 troops and there is still a separate US-led military coalition.

Mr Everts, his Nato counterpart, had also criticised the lack of strong central power in Afghanistan, which had meant a fragmented approach to running the country and an unhelpful division of responsibilities among the various states that have committed troops there.

As “super envoy”, Lord Ashdown would be expected to become the main point of contact between President Karzai and the international forces, EU policing mission and UN contingent, as well as co-ordinate reconstruction efforts. He would have to balance the need for "Afghanisation" with that of combating the Taleban insurgency.

A spokesman for Lord Ashdown declined to comment on whether an agreement had been reached on the post, nor on what precise powers he might enjoy. But Western diplomats said that he would have no oversight over the chain of command in Nato, which is leading the peace effort in Afghanistan.

His candidacy for the post, against other contenders including Joschka Fischer, the former German foreign minister, had been backed notably by the United States.

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