The surprise announcement was made at the Balkans summit in Sarajevo only a day after the prime minister completed his reshuffle.

If Mr Robertson were to picked to replace Javier Solana he would have to leave the Cabinet, prompting further movements within the government

It would also trigger a by-election in the Scottish MP's Hamilton South constituency.

The prime minister's spokesman said Mr Robertson's nomination had received "positive responses" from many world leaders at the summit.

The US, France, Italy and Spain are all said to have indicated their support.

Mr Blair said his defence secretary had "exactly the right mix of defence expertise and political and diplomatic skills to do an excellent job".

He added that The UK would be pressing "very hard" for Mr Robertson's appointment.

Mr Robertson was appointed to his current job following Labour's general election victory in 1997.

The 53-year-old MP was not one of the ministers who was expected to be moved in this week's reshuffle.

He was widely regarded as having had a "good war" during the Kosovo conflict and has proved himself to be an adept media performer.

Changing times at Nato

If the nomination was successful Mr Robertson would be taking over a time when the alliance is emerging from the greatest test in its 50-year history.

The Kosovo conflict was the first war that Nato had ever fought.

Mr Robertson, along with Mr Blair and Foreign Secretary Robin Cook, played a pivotal role in maintaining alliance unity throughout the campaign.

Mr Solana's four-year term ends in December. However, he is widely expected to leave a couple of months earlier to take up his new job as defence co-ordinator for the European Union.

Selecting a replacement is a matter of consensus, and several names have figured in press speculation.

They include German Defence Minister Rudolf Scharping. But the German Chancellor Gerhard Schroeder has said he wants to retain Mr Scharping where he is.