The President of the European Commission, Romano Prodi, says Britain's decision to stay out of the euro was a result of political rather than economic thinking.
By Justin Webb
BBC correspondent in Washington
Mr Prodi told the BBC Britain could not remain half in and half out of Europe.
He was speaking in Washington on the eve of the latest twice-yearly EU-US summit.
Prodi: Still time to join Europe
Mr Prodi said the UK Government's decision on the euro had disappointed him and was a signal of deep political problems.
He said Britain had a chance to swim in the European sea forever - that chance had not been taken.
Mr Prodi acknowledged that UK Prime Minister Tony Blair might have had a tough time winning a referendum but he said he did not know whether the prime minister's decision was due to political wisdom or a lack of courage.
Rift over Hamas
Later, Mr Prodi and Costas Simitis, the prime minister of Greece, current holders of the EU presidency, will hold talks with President George W Bush.
The two sides are expected to reaffirm a joint commitment to halting the spread of weapons of mass destruction, but there will be sharp disagreements on several subjects, including the treatment of the radical Palestinian group, Hamas.
The president will put pressure on the Europeans to abandon their distinction between the armed and political branches of Hamas, which the United States and Britain consider to be a single terrorist organisation.