A euro-vote would be "the biggest mistake of Tony Blair's career", warns Britain's best selling tabloid editor today.

Sun editor David Yelland has ruled out backing for the single currency this side of the next general election but insisted the paper's opposition to the euro is not fixed in stone.

The Murdoch-owned paper has in the past claimed that its backing for either Conservative or Labour has swung elections - a power Number 10 is said to have some faith in.

"The economic tests can be fixed and you must remember the Sun has never said never but [Blair] would have to persuade readers that he would reform Europe from the inside," said Yelland.

"I can't see the Sun campaigning for the euro ever; [Blair's] best hope - at some far off time in the future - might be that we simply don't oppose it."