Pro-euro ministers are set to lobby the prime minister over the date of a referendum on the single currency.

Ministers who want to have a vote on the euro in this parliament have already met to discuss how to persuade Tony Blair to prevent the chancellor ruling out a euro referendum before the next general election.

They are said to include senior ministers such as trade secretary Patricia Hewitt.

The pro-euro lobby has stepped up its campaigning with the release of an NOP poll for the Labour Movement for Europe showing 45 per cent in favour of a referendum now and 55 per cent for joining the single currency.

Former minister Peter Mandelson has challenged euro-sceptics arguing the government "must not shy away from this hardest of decisions".

Writing in the Sun, the former Northern Ireland secretary claimed staying outside the eurozone will mean Britain missing out on inward investment.

"The chancellor is right to be cautious. But in addition to the risks of going into the single currency - and there are risks - there are also risks of staying out," he said.

"And those costs could mean all the difference to how many good, well-paid jobs we have in Britain when overseas businesses decide where to invest in future."

Mandelson warned that Britain will be unable to stay outside Europe or influence reform of the EU.

"We would disappear into an economic deep black hole if we were to try to go it alone in today's independent world," he warned.

"That's why we need to ensure Europe goes in the right direction. The way to get the right answers is by being in there...not limping along several paces behind."

Meanwhile, Lord Radice, chairman of the Lords sub-committee on economic and financial affairs, has warned of the political costs of staying out of the euro.

Writing in the FT, he said that the five tests are "being deliberately used by the Treasury as a smokescreen for keeping Britain out indefinitely".

"If Mr Blair is to retain his credibility as a pro-European prime minister, he cannot allow British interests in Europe to be scuppered by his chancellor," he said.

The chancellor has already briefed the prime minister on the reasons behind his decision to block a referendum and is likely to set out his assessment to the House of Commons within the next fortnight.