President-elect Barack Obama plans to crack down on international tax havens, including Jersey, Guernsey and the Isle of Man, within weeks of taking power in January, putting him on a collision course with Gordon Brown.

There is growing international pressure to outlaw the secretive practices of tax havens as a key part of reforms to the world's battered financial system, as the leaders of the world's 20 most powerful economies gather for a major conference in Washington next weekend.

Britain has been notably lukewarm, but Obama, whose approval will be key to any reform package over the next 12 months, was one of the signatories of the Stop Tax Haven Abuse Act, legislation put to Congress last year that blacklisted Jersey, Guernsey and 32 other jurisdictions. Key aides to Obama said he will introduce a similar law as part of a wide-ranging revenue-raising and tax-reform package, within weeks of taking power.

Obama advisors estimate the measure could raise at least $50bn (£32bn) per year in lost US tax revenues, and Washington sources say leading accountancy firms have already hired lobbyists in anticipation of a fierce battle to water down the proposals.

Key measures are likely to include: revealing the beneficial owners of secretive trusts; prohibiting accountants from charging fees on specific tax services; and identifying 'offshore secrecy jurisdictions' that 'unreasonably restrict US tax authorities from obtaining needed information'. The measures could end years of financial secrecy that have protected the super-rich and international businesses as they move money from one jurisdiction to another.

Joe Guttentag, deputy assistant secretary for international tax in the Clinton administration and a key figure in the Obama campaign, is likely to drive the policy through, along with Professor Reuven Avi-Yonah, who helped frame the act. 'It is expected that something like this will happen,' Avi-Yonah told The Observer. 'There is a sense that if you can raise revenue by doing this, it will not be controversial.'

The measure comes as the UK faces international condemnation for blocking moves in the United Nations to upgrade its tax committee to intergovernmental status.

Brown has been keen to portray himself as the leader of efforts to reform the global financial system, boosting his credibility at home and distracting attention from the looming recession.

But as the Prime Minister prepares to set out his proposals for next weekend's conference in a speech at the Guildhall tomorrow night, anti-poverty campaigners will stage a noisy protest, urging him to 'call time on global greed'.

They fear Brown is too wedded to the light-touch regulation New Labour has championed for the past decade to be in the vanguard of a new economic system. 'Brown seems to have spent the past two weeks resuscitating the International Monetary Fund and refilling its coffers, so that it can lend on the same basis as the past 20 years,' said Nick Dearden, director of the Jubilee debt campaign. 'He doesn't seem to have done any soul-searching about how this crisis began.'