Ashton to leave EU foreign policy job next year

She added that the job is physically taxing.

"It's quite hard and there's a lot of travel and a lot of sitting on planes. My dear friend Hilary Clinton [the US ex-secretary of state] and I talked about this a few times. It is exhausting at times," she said.

Ashton's mandate is due to end on 1 December 2014.

It is unclear what she meant by "no possibility" of staying on, as there is nothing in the EU treaty which says she could not be reappointed.

There is speculation that her successor could be Nato chief Anders Fogh Rasmussen. But he says he has no interest in it.

Past candidates for the role, such as Swedish foreign minister Carl Bildt, could also come back into the race.

But one EU diplomat noted that the new foreign policy chief will be chosen as part of a "package" of EU top jobs designed to please big member states and political groups, not on personal merit.

The contact added that with so much time to go before December 2014, it would be "inadvisable" for any candidate to come out now.

Ashton said her main legacy will be the creation of the European External Action Service as an institution.

She said she did it at a tricky time, referring to the EU financial crisis and to the Arab Spring, and that it was hard to combine institution-building with diplomacy.

"I've often described it as trying to fly a plane when you are building the wings at the same time," she noted.

Bashed in the past on a range of issues - poor management skills, chaotic planning of foreign ministers' meetings, lack of courage and charisma - one EU diplomat told EUobserver that her early announcement on stepping down is itself a mistake.

"I think it weakens her. I think it was unnecessary," the contact noted.

But for her part, Ashton said she has grown a thick skin.

"It's not about the criticism. I've had enough of that. I'm not worried about that anymore," she noted.

She acknowledged there are limitations to how much EU countries really want to co-operate on foreign policy, especially in military matters.

"These are national [military] services and the decision to deploy and put people in harm's way are national sovereign decisions," she said.

But she denied reports that the European Commission has tried to undermine her work in order to defend its turf.

"He [commission head Jose Manuel Barroso] has always been hugely supportive," she said.

"Why would he be jealous? Blimey. No. I don't think he's jealous at all. I think he really wants this to succeed," she added.