Paddy Ashdown, the former leader of the Liberal Democrats, will shortly be named the United Nations' new envoy to Afghanistan.

The appointment of a high-profile figure to the position is designed to address poor co-ordination among multinational agencies, the military and the Afghan government that has threatened to jeopardise the international effort in the country.

According to diplomats and international officials, a proposal was debated to give Lord Ashdown a position simultaneously with the UN, the Nato alliance and the European Union - so-called "triple hatting" - but was rejected. He will have a broader mandate, however, than his predecessor, Tom Koenigs.

Lord Ashdown will have a close working relationship with Nato but will have to "rely on charisma and the force of his personality" to push through greater coordination, a senior UN diplomat in Kabul said.

Lord Ashdown, 66, who was the UN's highest representative in Bosnia from 2002 to 2006, is to take the post after several months of negotiation with the Afghan government, the UN and Nato member states.

"He wanted to be clear about his mandate and wanted to be sure he had a substantive job," said a senior British government official.

"You can't co-ordinate all the strands that need to be co-ordinated in Afghanistan if you don't have a sufficiently strong mandate to do so. The role that he will have has had to be agreed with a wide number of stakeholders, not the least of which is President Hamid Karzai himself."

The former royal marine has strong US backing. His experience of a country aiming to recover from conflict, coupled with a background as a military officer and a politician, are regarded as giving him strong credentials for the position.

According to officials, Mr Karzai has been concerned that a UN super-envoy would undermine Afghan sovereignty as the country heads to presidential and parliamentary elections next year.

The UN secretariat and some member states have also been concerned that the UN might become identified too closely with the Nato military alliance. Already UN officials in Kabul are alarmed that the organisation is no longer seen as an impartial provider of humanitarian aid.

International hopes that the Taliban had hitherto avoided targeting civilian soft targets have been shattered by Monday's attack on the Serena hotel in Kabul. Taliban officials have threatened more attacks, including targets such as restaurants favoured by Kabul's large expatriate community.

There is also concern in Kabul that Lord Ashdown's appointment will heighten anti-British sentiment among ordinary people. One senior European diplomat warned Lord Ashdown that he should expect such anti-pathy.