“If Gordon Brown has nothing to hide he could break the habit of a lifetime and give some straight answers, something his spokesman notably failed to do when asked about exactly this point.”

The Tories fear that any move towards preparing Britain to join the euro could send interest rates soaring.

Mr Hague added: “Keeping the pound is vital for Britain’s economic future. We need interest rates that are right for Britain, not the rest of Europe.

“There are no circumstances in which the next Conservative Government will propose joining the euro.”

Whitehall sources also revealed that the idea of Britain joining the euro has been raised with the Prime Minister by officials in discussions about measures to tackle the recession.

Mr Brown was reportedly enraged over speculation that the move was under consideration. He is understood to fear a massive public backlash.

Britain’s potential membership of the euro has been a dead issue at Westminster for nearly a decade.

But it erupted on Monday after Mr Barroso divulged that he had  discussed the issue with senior British politicians.

Mr Barroso admitted: “I know the majority are still opposed, but the people who matter are thinking about it.”

His remarks provoked outrage, with critics suggesting that Eurocrats do not see British voters as “people who matter”.