Leading article: A rebuff from Mr Karzai that could cost Afghans dear

The appointment of an international representative is usually sewn up discreetly in advance and announced as an accomplished fact. So the public disagreement that has erupted over the post of special envoy to Afghanistan breaks almost every rule in the diplomatic book. Ten days ago, it appeared that Lord Ashdown had been offered the post and had accepted. It seemed an admirable fit. Here was a job that needed doing and an individual eminently well qualified to do it. There were just a few ends to be tied up.

At the weekend, after rumours that all was not going smoothly, Lord Ashdown wrote to the UN Secretary-General to withdraw. He pointed out, reasonably enough, that he had not sought the job, that anyone who wanted to succeed had to have the support of the Afghan government, and that it was clear to him that "in Afghanistan, at least, the support necessary to do the job effectively does not exist".

It is reassuring to hear such forthright words from someone in public life. Lord Ashdown did not beat about the bush. If the Afghan government did not want him there, there was no point in his going. Therewith he ended his involvement.

That might be realistic, but it is a pity. As a former Marine commando of distinction and, more recently, the UN's High Representative and EU envoy to Bosnia, Lord Ashdown has earned a reputation as a competent and effective administrator in precisely the sort of conditions that pertain in Afghanistan. There are very few people with as much experience as he has of negotiating the difficult transition from civil war to peace. He would surely have had a contribution to make.

For Lord Ashdown to occupy the post might have also have helped the Government make its case for the continued military presence in Afghanistan. As British deaths in Afghanistan have risen, pertinent questions have been raised about the way in which a mandate to help with reconstruction has so quickly escalated into a combat mission. A persuasive advocate for the causes he takes on, Lord Ashdown might have been able to explain to an increasingly sceptical public, how this "mission creep" could be justified.

Opposition from Kabul, and from President Hamid Karzai personally, is clearly what scuppered the peer's appointment. The Afghan foreign minister said Lord Ashdown had at times been a "very controversial figure" in Bosnia. Another suggestion was that a strong administrator, such as Lord Ashdown, risked making President Karzai look weak. There is also speculation that his nationality was a problem, as Mr Karzai and others had blamed the British in Helmand province for letting the Taliban back in.

At the same time, however, the Afghan ambassador to the UN said his government's preferred candidate was another Briton, General John McColl. Currently Nato's deputy commander in Europe, the general headed the first international force in Afghanistan six years ago. The difference between him and Lord Ashdown – one a low-key soldier, the other now a high-profile administrator – may help to explain the Afghan government's preference. President Karzai may also have been unnerved by Lord Ashdown's trenchant criticism of Western mistakes in Afghanistan and his warning about the wider risks if Afghanistan was "lost".

It is understandable that Mr Karzai would not acquiesce in anything that he feared might jeopardise his own authority or destabilise his country further. In changing his mind about Lord Ashdown, however, he may have rejected not only one of the few people who still believes the country can be rescued from the brink, but perhaps the only one with a real chance of achieving that.