London  - US President Barack Obama arrived in London Tuesday for the Group of 20 (G20) summit of the world's big economic powers at the start of his first major foray onto the global stage.

After he and his wife, Michelle, touched down at London's Stansted Airport they travelled by helicopter to the US ambassador's elegant residence in Regent's Park in central London.

But even before the G20 summit started, key leaders were playing down expectations of what might be achieved at the meeting, saying that another gathering will probably be necessary to implement measures to combat the global recession.

Speaking ahead of the summit, European Commission President Jose Manuel Barroso said he was not expecting "miracles" from the G20.

"The G20 will not end this crisis overnight. But it can and must make a difference," Barroso said, adding that more summits will be needed on the crisis, including at least one more this year.

Obama's trip comes against the backdrop of a deepening sense of economic gloom with the leaders attending a summit expected to agree to a major revamp of the world financial system as well as steps to bolster coordination between governments in combating the downturn.

"The president and America are going to listen in London, as well as to lead," said White House spokesman Robert Gibbs.

But ahead of the summit, the World Bank on Tuesday issued one of its bleakest set of forecasts predicting that the global economy will shrink by 1.7 per cent this year in the wake of a collapse in trade and the global financial crisis.

Despite attempts by G20 summit host British Prime Minister Gordon Brown to forge a consensus ahead of the summit, tensions have flared in the build up to the meeting with French President Nicolas Sarkozy threatening to "walk out" if no concrete measures were agreed on global regulation.

"If there is no progress in London, there will be an empty chair. I will get up and leave," Sarkozy said in an aside during a recent meeting of his cabinet, the daily Le Figaro reported on Tuesday.

But a spokesman for Brown played down the Sarkozy comments saying Brown had had a "very constructive discussion" with the French president Monday and was looking forward to him playing a "very full role" at the meeting.

Brown Tuesday briefed the British cabinet on preparations of the summit and the degree of consensus that was emerging.

While a "huge amount" of progress had been made towards reaching a consensus, there remained "clearly more to do," said Brown's spokesman.

"The prime minister is working the phones very hard at the moment," he said.

However, with the summit marking Obama's first major trip outside North America he is likely to be a major focus of the meeting, which is to be launched Wednesday at a reception at Buckingham Palace.

Apart from marking a new era in trans-Atlantic relations, Thursday's G20 summit will also represent the first opportunity that many world leaders have had to meet the new US president.

Obama is scheduled to meet with leaders of China, India, Saudi Arabia - three of the world's key emerging economies - as well as Russian President Dmitry Medvedev.

After the London summit, he is to attend the 60th anniversary NATO summit to be held in Germany and France, where he is due to meet both German Chancellor Angela Merkel and Nicolas Sarkozy.

Obama's eight-day visit to Europe will also take him to Prague for a US-European Union summit and then on to Turkey.

The White House is hoping his trip will help to lay aside the fraught relations that emerged between Washington and Europe as well as the Muslim world under US president George W Bush.

However, the build up to Obama's departure and the London summit has been accompanied by tensions between Washington and its European allies over the White House's push for fresh fiscal measures to spur global economic growth.

Spearheaded by Merkel, Europe remains sceptical about throwing more money at the recession, arguing that it will take some time for the measures introduced to take effect and that the focus should now be on financial reform.

But none of the government chiefs attending the London summit will want to see it fail in the midst of the world's biggest financial crisis since the Great Depression.

Least of all Obama, who will to be keen to ensure his first steps on the world diplomatic stage do not end on a sour note.

So instead of new concrete budget measures to help haul the global economy onto its feet, the G20 leaders are likely to take their cue from their finance ministers' meeting this month and simple declare they will do what ever is necessary to fight the recession. (dpa)