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While the departure of US General Stanley McChrystal is making headlines around the world, another straight-talker who made a key contribution in Afghanistan appears to be gone for good - though his departure has been an understated affair.

Sir Sherard Cowper-Coles, the UK's most senior diplomat in Afghanistan, has begun a period of "extended leave".

His unexpected departure comes at a time of mounting British casualties and growing doubts about the US and Nato strategy.

The special envoy himself is believed to be one of the doubters.

On a visit to Pakistan on Wednesday, Foreign Secretary William Hague praised his contribution but appeared to rule out a return to his post.

"He's having some hard-won leave," he said in Islamabad.

"He's given an immense effort. We will let him have that leave and then we'll take it forward from there. As regards what he will do in the future, we will consider that over the next few weeks."

When pressed on whether or not Sir Sherard would be returning to his post after his leave, the foreign secretary replied: "People can draw their own conclusions from what I have said."

The career diplomat has handled a number of tricky assignments in the past - serving as ambassador in both Israel and Saudi Arabia.

It is testament to his political and linguistic skills that he could move from one to the other.

But Sir Sherard (he was knighted in 2004) is known to speak out, rather than whisper. And some believe that was the reason for his departure.

During his time in Afghanistan he has pressed for talks with the Taliban to get greater priority, and is reported to have clashed with senior Nato and US officials over his approach.

According to one seasoned observer of the Afghan quagmire, the envoy undermined himself by getting involved in too many disputes.

Sherard Cowper-Coles says he has taken leave because the job is done.

He believes he has gotten across the key message - that the Taliban and other insurgents have to be part of the solution.