The ECB, based in Frankfurt, is in charge of the single currency

Denmark, Sweden, the UK and the 10 new entrants are outside the euro zone.

Interest rates in the euro zone are set by the European Central Bank; critics say this "one size fits all" policy has harmed economic growth.

Labour says it will only join the euro when it has judged conditions are right for the UK economy.

The Conservatives would stay out of the euro, while the Lib Dems favour euro membership in principle.