Labour leader Ed Miliband has issued a stern challenge to the Isle of Man and other offshore jurisdictions, in a pre-election drive against tax avoidance.
Mr Miliband says a Labour government would demand to 'see the books' of Crown Dependencies and Overseas Territories within six months if it wins May's general election.
In an interview with The Guardian, Mr Miliband says he has written to leaders in jurisdictions including Bermuda, Jersey, Gibraltar and the Cayman Islands, putting them on notice they would have to produce a publicly accessible register of the ownership of offshore companies.
If they failed, the labour leader says he will ask the Organisation for Economic Co-operation and Development (OECD) to put them on a tax haven blacklist.
Leaders of G20, representing the worlds largest economies, have previously agreed to impose sanctions against jurisdictions blacklisted by the OECD.
In recent years, the Manx government has signed a series of tax information exchange agreements with foreign countries including the UK, and says it is committed to improving tax transparency in line with international standards.
At present, the beneficial ownership of Manx-registered companies is available to certain authorities, on request.
Last year, government held a consultation on whether a central registry of company ownership should be set up - responses and options for the future are still being considered, and an action plan is promised.
Under the FATCA regime, to which the Island is signed-up, financial information on foreign nationals is already relayed back to tax authorities in their country of origin.