French President Nicolas Sarkozy, who has threatened to walk out of the meeting, said that new financial regulation was a "non-negotiable goal".
In London's financial district, police and protesters have clashed. A protester died after he collapsed.

Meanwhile, US President Barack Obama and his wife Michelle have met the Queen at Buckingham Palace. The G20 leaders attended a state dinner at Downing Street, where they began to hammer out a final agreement. They were served a meal of Welsh lamb, Jersey Royal new potatoes and asparagus cooked by celebrity chef Jamie Oliver.

Downing Street today signalled that a further meeting of the G20 could be necessary later this year because this week's summit in London will leave significant issues unresolved.

The prime minister's spokesman said that the G20 summit, which will take place now, was being seen as part of a "process" and that it was a process "nearer to its beginning than its end".

The comments, which appear to downplay expectations of what will be achieved at the meeting, came after it emerged that any spending decisions would be deferred until some point in the future.

Yesterday, Kevin Rudd, the Australian prime minister, said it was now up to the International Monetary Fund to determine how much additional support the world economy would need next year, and that there had never been any expectation that the decisions on that package would be taken in London.

"That was never the intention," he said. "A mechanism has been established for us to reflect on for what we need for the future. There will be a further summit, well in time for 2010, I assume, which will actually look at what metrics, what numbers, will be needed then."

Silvio Berlusconi, the Italian prime minister, has already suggested the G20 leaders meet again in Sardinia in July at the end of the scheduled G8 summit. Today Downing Street said that the government could see "merits" in a further G20 meeting later this year, although the prime minister's spokesman said that Britain would not expect to chair it. The spokesman said that this was something that would be discussed by the leaders on Thursday.

The prime minister said that a great deal had already been achieved as a consequence of the decision to hold a meeting of G20 leaders in London. The G20 normally only meets at finance minster level, and Thursday's meeting will mark only the second time that it has assembled at heads of government level.

"Already the work leading to the summit has generated action on a wide range of issues, on a scale that would normally have taken years to achieve," Brown wrote.

"We see it very much as a process, rather than an individual meeting. It is a process that is nearer to its beginning than its end," the spokesman said.

One problem facing Brown as he prepared for the summit was that other European leaders are not prepared to make further injections of money into their economies now, as the Americans would like them to do. In an interview with the Financial Times today, Barack Obama appears to concede that this has been a problem.

"In all countries there is an understandable tension between the steps that are needed to kick-start the economy and the fact that many of these steps are very expensive and taxpayers have a healthy scepticism about spending too much of their money, particularly when it is perceived that some of the money is being spent not on them but on others who they perceive may have helped precipitate the crisis," the US president said.

But in his interview, Obama also stressed the need for leaders attending the G20 to "deliver a strong message of unity" for the sake of the world economy. Brown made a similar point in his Evening Standard article, saying that the London summit was "the chance to show that we can work together".

This morning Brown had talks with his Australian counterpart in Downing Street. Afterwards Brown said that some measures due to be agreed on Thursday, such as a clampdown on tax havens, would have been "unthinkable a year ago".