The prime minister has given offshore tax havens a time limit by which they must meet international standards - or risk sanctions.
Gordon Brown has written to British crown dependencies and overseas territories warning them to open up their tax practices to international scrutiny.
The prime minister gave the territories - including Bermuda, the Cayman Islands and Gibraltar - six months to adhere to international standards or face sanctions. The tax centres are currently on a "grey list" compiled by the Organisation for Economic Co-operation and Development (OECD), made up of locations which have pledged to sign up to information-sharing deals.
Mr Brown has also written to Guernsey, the Isle of Man and Jersey, which are on the OECD "white list", made up of those centres which have "substantially implemented" measures. He welcomed their progress but warned that more steps towards transparency must be taken.
Four countries which were on the OECD "black list" - namely Costa Rica, Malaysia, the Philippines and Uruguay - have since been moved to the "grey list" after they pledged reforms.
The issue of offshore tax havens was raised at the recent G20 summit in London, when leaders pledged to tackle the problem of tax-avoiding businesses as part of a wider package of measures to combat the financial crisis. US president Barack Obama also made clear in his election campaign that he would crack down on tax havens during his time in office.
