Mr Brown is focusing attention on offshore financial centres in Britain's overseas territories

Mr Brown, who's backing a new global assault on tax avoidance, has written to the Organisation for Economic Cooperation and Development (OECD) about the need to build on recent international breakthroughs on bank secrecy.

He has said that that is necessary to make further advances in the fight against harmful tax practices.

The British leader told the OECD that his priorities were to address urgently the issue of tax avoidance, as well as ensure "developing countries can benefit from the transparency we are now achieving".

He wants the OECD to relaunch its drive to outlaw harmful tax competition, a move he believes can end tax abuses by big multinationals that deprive the public purse of hundreds of millions of pounds.

Prime Minister Brown has been underscoring his determination to make sure tax havens with British links are held to account.

He was expected to call for reform from Bermuda, the Cayman Islands, the Turks and Caicos Islands, and Gibraltar after securing agreement from G20 leaders for global action.

UK dependencies appear on the OECD's "grey list" of jurisdictions which are committed to international standards but have yet to implement them.

"We have got to make sure that the tax havens that have got some relationship with Britain are conducting their affairs in such a way to justify the decisions that we made at the G20 that change is going to happen," Gordon Brown said while denying that the move was motivated by governments seeking a tax grab.

He said it was about "people in countries which have not upheld standards that we have been prepared to uphold, now being held to account".

Caribbean offshore financial centres, including the Cayman Islands, have said they are moving to ensure full compliance with regulations that promote transparency and the sharing of tax information.