Some of the world's biggest campaigners and judicial investigators against corruption are writing to David Cameron asking him to take action to require G8 countries to publish company registers setting out the ultimate owners of firms, rather than their shell companies. The prime minister is chairing the summit of the leaders of the G8 industrialised countries next week in Northern Ireland.

The issue of public registers and beneficial ownership is rapidly becoming one of the most difficult for Cameron to address at the summit, in the face of hostility from G8 members Russia, Canada and the US. Proper company registers – ideally available to the public and not just tax authorities – are seen by campaigners as necessary to stop corruption and money laundering, and to identify individual and corporate tax evaders.

Cameron has made tax transparency the central theme of the summit starting next Monday in Northern Ireland.

The letter has been signed by John Githongo, former head of the Kenyan anti-corruption agency; Judge Richard Goldstone, former chief prosecutor of the UN criminal tribunals for the former Yugoslavia and Rwanda; Eva Joly, French MEP; and Mark Pieth, chairman of the OECD working group on bribery, who was appointed by Fifa to examine corruption in football.

The letter comes as Cameron prepares to make a set piece speech on Monday on the role of Britain in the world ahead of the G8 summit.

He has also been given a political boost by the Cayman Islands agreeing to sign the OECD multilateral convention on tax transparency and information, the most important of the British overseas territories to do so. The Caymans has also agreed to attend a meeting in London organised by Cameron, at which he is to urge them to do more to end their tax haven status. Some of the Crown dependencies have already agreed to attend.

Downing Street has been concerned that the leak of Cameron's plan to call in the overseas territories would lead to a backlash and refusal to attend.

The letter by the 18 so-called "corruption hunters" claims: "Countries must commit to take action to prevent anonymous companies from being used to hide criminal activity. Corrupt politicians, tax evaders, and organised criminals all use complex webs of shell companies to hide and launder stolen money.

"We believe that part of the solution is for governments to require existing company registers to collect information on the ultimate owners of all companies. To have the most impact, this information should be in the public domain. This would help law enforcement chase down money launderers, but it will also enable citizens, journalists and civil society to hold companies to account for their actions. It is also important that this should be extended to cover G8 members' offshore territories."

They add: "Governments need to send a strong signal to their financial institutions that it is unacceptable to do business with corrupt politicians. Banks that are found to breach anti-money laundering rules should face strong, dissuasive penalties. These should also be applied to senior executives who turn a blind eye to dirty money passing through their banks or those who have neglected their duties to ensure that their organisations have the required systems in place to prevent money laundering."

The Cayman Islands Premier, the Hon Alden McLaughlin, said in a statement at the weekend: "Cayman has engaged in substantive discussions with HM Treasury on the particulars of the convention. We are satisfied that the extension of the convention to our islands will be done in accordance with the UK's recognition of Cayman's fiscal autonomy, and the well-established principle that countries have the prerogative to set their own tax rates."

He added: "We agree with the UK's statement that there is 'no point in dealing with tax evasion in one country, if the problem is simply displaced to another'. With this in mind, we also agree that there should be equitable adherence, including within the G8 countries, to global tax and transparency standards. This will set the foundation for full and effective participation, by all countries, in the true spirit of these efforts."

The If campaign, a coalition of 200 charities campaigning against world poverty, welcomed the Caymans statement. "This is the first hole in the wall of secrecy surrounding Britain's overseas territories.

"David Cameron now has a real opportunity to show that all the UK's tax havens – both overseas territories and crown dependencies – can reform and help end the scandal of tax-dodging. We need quick action to deliver a deal at next week's G8 to ensure poor countries can collect the billions in missing tax that would help them end hunger."