British Prime Minister David Cameron will push for global action on corruption at a summit Thursday attended by the leaders of Nigeria and Afghanistan, countries he was caught describing as "fantastically corrupt".

Nigerian President Muhammadu Buhari and Afghanistan's Ashraf Ghani are attending the London summit alongside the Colombian president, US Secretary of State John Kerry, NGOs and the leaders of the World Bank and IMF.

Cameron has described corruption as "an enemy of progress and the root of so many of the world's problems" and says the summit, which comes amid public outrage over the Panama Papers leaks, will push it to the top of the global agenda.

But he risked embarrassment when he was caught on camera on Tuesday telling Queen Elizabeth II that leaders of some "fantastically corrupt countries" were attending, citing Nigeria and Afghanistan as "possibly the most corrupt".

Downing Street later said both Buhari and Ghani had admitted the challenges their countries face and were invited because of their commitment to tackling them.

- Anger over Panama leaks -

Campaigners are hoping for concrete action from the summit in response to public anger sparked by a huge leak of documents from a Panamanian law firm, which revealed in detail how the rich and powerful hide their money.

Activists want governments to expose the beneficiaries of anonymous companies used to move money without detection, and take further action to reduce the secrecy of offshore tax havens where they are incorporated.

There is also speculation that Cameron will announce measures to tackle money laundering through London's luxury property market, following his declaration last year that there was "no place for dirty money in Britain".

A British official said the summit declaration would commit signatories "to expose corruption wherever it is found, to pursue and punish those who perpetrate, facilitate or are complicit in it".

But The Times reported that a draft being circulated had been watered down following opposition from some countries, with one passage stating there would be "no impunity for the corrupt" removed.

The Russian foreign ministry, which is sending deputy foreign minister Oleg Syromolotov, has already indicated that any final agreement may not be binding.

Robert Barrington, UK executive director of Transparency International, said Cameron should "not give in" to attempts to limit the summit's ambitions.

"The prime minister has created a platform for governments that are serious about fighting corruption, and now it is up to others to show they share the same ambition," he said.

Cameron has been outspoken on the need to tackle aggressive tax avoidance and evasion and global corruption since raising the issue at the G8 in Britain in 2013.

He was embarrassed last month when his late father's Bahamas-based investment fund was named in the Panama leaks, although he insisted it was entirely legitimate.

- 'Clean out your own backyard' -

The Panama leaks also shone a spotlight on the role played by Britain's overseas tax havens, and London-based lawyers and accountants, in promoting financial secrecy.

Britain will introduce a public register of who ultimately benefits from British-registered companies next month, a step towards greater transparency.

But campaigners want London to extend the measure to its overseas territories, forcing them if necessary.

"If you're going to stake out a claim to be a leader, clean out your own backyard," said John Christensen of the Tax Justice Network advocacy group.

More than 300 economists, including former IMF economist Oliver Blanchard and France's Thomas Piketty, this week urged world leaders to end offshore financial secrecy, saying tax havens "serve no useful economic purpose".

In an open letter to Cameron ahead of the summit, almost 100 Nigerian civil society groups also warned Britain was a "safe haven for our corrupt individuals".

"This summit is an opportunity for the major financial centres to finally acknowledge the role they are playing in perpetuating corruption," they wrote.

Representatives of the Cayman Islands, a British overseas territory, are expected to attend the summit -- but it is not just offshore territories that are under pressure.

The United States, represented by Kerry, is also being urged to address the lack of transparency in US states such as Delaware and Wyoming, where anonymous companies can be created for just a few hundred dollars.

Ahead of the summit, the Commonwealth announced plans to develop an international standard or mark that could be granted to firms adhering to best practice in procurement, to help boost the fight against corruption. afp