An influential committee of MPs has warned of "obstacles" to British membership of the European single currency.

In a report published on Monday, the Commons Treasury select committee also said that if Britain rules out membership then a "window of opportunity" to influence the workings of the European Central Bank could close.

With Gordon Brown to announce the results of the five tests on memberships before the first week of June, the report will be seen as strengthening the hands of sceptics in the Treasury.

Reports suggest the chancellor has agreed with Tony Blair that the tests have not yet been met.

However, the prime minister is believed to have insisted on keeping open the option of reviewing them again later in the parliament.

But ahead of the announcement, the cross-party committee of MPs said that proposed changes to the ECB ahead of the expansion of the European Union to central and eastern Europe would weaken Britain's influence if it joined.

The changes mean that Britain would have no say in one in five interest rate-setting decisions - although the same rotational system would also apply to France, Germany, Italy and Spain.

The committee said that the new arrangement was not an "optimal solution" to the problems of EU enlargement and warned that it could have implications for British membership.

"We think the prospect of UK exclusion from 20 per cent of ECB interest rate votes could prove to be an obstacle to entry," the report concluded.

But it added that ruling out membership would have "significant political implications" for Britain's ability to influence economic policy in Europe.

"There may currently be a 'window of opportunity' for influencing debate on reforms within the euro system, which may close if the UK decides not to join."

The committee also highlighted concerns that a government failure to make a definitive statement on its position could lead to turbulence on the money markets.

"Such a rolling re-run of the five tests could lead to uncertainty in the British economy," it said.

The Conservatives seized on the report as proof that the government lacked a coherent policy on the euro.

"This report does not seek to reach a conclusion on whether Britain should join the euro. But its findings strike at the heart of government policy," said Michael Howard.

The shadow chancellor said the five tests could not be met "clearly and unambiguously" while reconsideration of the tests at a later date would undermine the economy.

He added that "this government's decision is more likely to result from the triumph of one political faction over another".

"It is vital that Britain can set the interest rates that are right for us. Otherwise jobs and prosperity are at risk," said Howard.

"It is time for the government to forget about the euro and focus on standards in public services instead."