Failure to set a deadline for joining the euro could result in the collapse of the Britain in Europe campaign group, Tony Blair has been warned.

As the government prepares to reveal whether its "five tests" for membership of the single currency have been met, pressure is mounting for the option of a second review later in this parliament to be kept open.

The prime minister and several senior Cabinet ministers are believed to favour a re-evaluation of the tests before the next general election if, as expected, Gordon Brown rules that so far they have not been met.

With polls continuing to suggest that a significant majority of voters are sceptical about joining the euro, the government would rely on the cross-party Britain in Europe group to play a key role in any referendum campaign.

But in an interview with the Guardian on Saturday its campaign director, Simon Buckby, said that "warm words" from the government would no longer be enough.

"How could I motivate my staff, invigorate senior businessmen, trade unionists and voluntary organisations if the government statement did not set out a clear route map to membership of the euro," he asked.

"Business wants to know it is the government's intention, not just their hope, that we will join the euro in this parliament.

"There can be no more wait and see - there has to be a game plan, with a transparent political strategy, to secure membership."

Buckby also warned that failure to set a timetable for euro membership would damage Britain's influence inside the European Union.

He said that the prime minister would be seen as "weak and dithering" by other European leaders.

Buckby's comments will be taken as a further sign that those who favour euro membership believe they are losing the argument against a more sceptical Treasury.

With the chancellor due to make his statement by the first week of June, speculation about its contents will only intensify over the coming weeks.