rime Minister David Cameron to hold first sit-down with President Barack Obama at G8

The Prime Minister and the President are expected to discuss BP and the Gulf of Mexico oil spill

President Obama has urged world leaders to follow his lead on regulatory reform at the G20 summit, which is also taking place in Canada and will deal with debt mountains that threaten the global recovery after the worst financial crisis in 80 years.

The Prime Minister and the President are expected to discuss BP and the Gulf of Mexico oil spill, amid fears that the costs of the disaster could destroy the massive company.

BP's shares have fallen to a 14-year low and $100 billion has been wiped off its market value since the start of the biggest oil spill in US history, which has now lasted for more than two months.

Mr Cameron told a Canadian broadcaster that there must be clarity over BP's costs for the spill to avoid the destruction of the company.

"It is also in all our long-term interests that there is some clarity, some finality, to all of this, so that we don't at the same time see the destruction of a company that is important for all our interests," he said.

"I believe it is in both Britain and America's interests that BP remains a strong and stable company."

The company says it has paid out $2.35 billion so far in cleanup and compensation costs for the spill. That does not include the $20 billion oil spill fund it has agreed to set up or the billions of dollars it will have to pay in fines.

The pair are also expected to discuss Afghanistan on the sidelines of the summit after the resignation of the general in charge of the war, Stanley McChrystal, and his replacement by General David Petraeus.

Mr Cameron told broadcasters when he arrived in Canada that he wants to see British troops home by 2015.

The two leaders are likely to discuss strategy ahead of this year's "surge" in troop numbers, as soldiers prepare for the annual summer increase in fighting and an expected jump in casualty numbers.

Mr Cameron has called on countries attending the G8 to stand by their aid pledges to poor nations.