The decision on whether the UK joins the euro will not solely be political and will go beyond the chancellor's much-trailed five economic tests, the prime minister said last night.

Returning from the Commonwealth meeting in Australia, Tony Blair told reporters that factors such as exchange rates would play a part in the final decision.

"There is an issue about the exchange rates when you come to the decision but that's for the future," said Blair.

He said that the five economic tests set out by the chancellor would "have to be passed" and stressed that no decision had yet been taken.

"The economic assessment has not been made," said the prime minister. "The economics are important and that's precisely why we have got the economic test."

Blair said that "people will watch to see whether economically it's in their interests" before deciding whether to back British membership of the euro.

But even then, Blair insisted that other factors - such as the rate of exchange at which the pound went into the eurozone - would enter into the equation.

Downing Street dismissed suggestions that the prime minister's remarks represented a change in the government's policy - which to date has simply referred to the five economic tests laid-down by the chancellor.

A spokesman said the prime minister's remarks were not intended to be "anything new" although his comments were being seen by some at Westminster as fresh evidence that senior ministers are blowing cold on British membership of the single currency.