The BBC's Justin Webb reports: "He now faces an in-tray with an even bigger set of challenges"

Mr Robertson said bringing new members into Nato would be among his priorities when he takes over from Spain's Javier Solana later in the year.

"We've made it absolutely clear that the door is not closed," he told a UK Ministry of Defence news conference.

Robertson: In the field in the Balkans

Another key challenge would be rebuilding ties with Russia after the Kosovo conflict, he said.

Mr Robertson said: "It's a daunting prospect to be asked to follow in the footsteps of so able a predecessor as Javier Solana.

"It's a great honour but also a formidable challenge to take on the leadership of the North Atlantic alliance in its 50th anniversary year.

"In Bosnia and Kosovo in the 1990s, Nato showed it was as needed as it had been in the four decades before."

The UK defence secretary also promised to strengthen Europe's input into Nato when he takes over. He referred to this as the major lesson he had learned during the Kosovo conflict.

He said: "The European countries spend about two-thirds of what the United States of America spends on defence but we do not have anything like two-thirds of the capability."

Blair 'delighted'

Nato's members met in Brussels on Wednesday to decide on a new boss for the alliance and unanimously agreed on Mr Robertson.

UK Prime Minister Tony Blair described himself as "delighted" after Nato confirmed its pick for the top job.

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"George Robertson has all the qualities to make a success of this important job," he said. "Nato could not be in better hands as the alliance enters the new millennium."

Until recently a relatively unknown British politician, Mr Robertson will now lead the world's most powerful military alliance.

Nato spokesman Jamie Shea on the three challenges George Robertson will face

The 53-year-old Hamilton South MP was proposed for the Nato top job by Mr Blair and his appointment has been welcomed by defence spokesmen for Britain's two main opposition parties.

He had the backing of the United States - the biggest and most influential Nato member - as well as Germany and France.

Mr Robertson emerged from the Kosovo conflict with increased international standing after his forthright warnings to Yugoslav President Slobodan Milosevic.

Javier Solana: Former chief leaves for EU defence job

But his elevation to Nato secretary-general presents his biggest challenge yet, as the alliance redefines it relationship with Russia and Eastern Europe.

US State Department spokesman James Rubin said: "The United States is pleased that he has been chosen.

"He is an extremely able minister who has served his government well, not only during the Kosovo conflict but also in the crucial debates on Nato's future that preceded the Washington summit."

But Mr Robertson is known to have been President Bill Clinton's second choice, after German Defence Minister Rudolf Scharping, who declined the job.

Reshuffle effect

Mr Robertson's appointment to Nato will mean a reshuffle within Mr Blair's Cabinet - a week after the prime minister unveiled changes to his government.

Watch George Robertson's news conference at the MoD

It will also trigger a by-election, as Mr Robertson will have to relinquish his central Scotland seat. He has a majority of nearly 16,000.

Mr Robertson has refused to be drawn on who would replace him as defence secretary, but Scottish Secretary Dr John Reid, a former defence minister, is a favourite.