Ministers have lined up to back policy on the European single currency following a row over the political or economic character of the government's assessment of euro-membership.

Labour Party chairman Charles Clarke told the Sunday Times that the UK's five economic tests would not lead to "an absolutely unarguable position" because economics "is not an exact science".

But the Treasury's chief secretary, Andrew Smith, reasserted the government's line that the tests must be met, saying it must be "clearly and unambiguously in the country's economic interests to join".

Meanwhile, chancellor Gordon Brown has indicated that a sixth test, the euro-sterling exchange rate, could be the biggest barrier to UK entry, according to the Guardian.

Commons leader Robin Cook told the BBC that Britain was playing a leading role in Europe "and if we want to continue that powerful role then it is going to be more challenging to do that if we are outside the inner club".

The comments came as two ICM polls found 73 per cent of the public in favour of retaining sterling, while a Sunday Times poll found the public warming to the euro and an online poll by YouGov found 52 per cent wanting to join the euro.