British prime minister David Cameron promises to hold a referendum on Britain's membership of the European Union.
If his government is re-elected in 2015, he said a simple "in or out" referendum will be held by 2017, following a re-negotiation of the terms of British membership.
Mr Cameron stressed he wants a relationship that keeps Britain in the EU, but said a growing gap between the EU and its citizens is increasingly being felt in Britain.
The news was welcomed by Eurosceptics who have long campaigned for a vote. But France and Germany both warned that Britain cannot "cherry pick" EU membership.
German foreign minister Guido Westerwelle said Europe needed more, not less, integration.
In a long-awaited speech on Wednesday, Mr Cameron said:
"It is time for the British people to have their say," he said. "It is time to settle this European question in British politics. I say to the British people: this will be your decision."
However, Mr Cameron did not spell out what powers he would like to see Britain take back as part of a new settlement or what would happen if the negotiations did not go his way.
The Conservative Party leader has been under pressure from many MPs to give a binding commitment to a vote on Europe.
He dismissed warnings this could imperil Britain's diplomatic and economic prospects and alienate its allies.
Mr Cameron said Britain did not want to pull up the drawbridge and retreat from the world but public disillusionment with the EU was at "an all-time high".
Mayor of London Boris Johnson said the British people had not been consulted on Europe since 1975, and it was "high time" they were.
The speech, which has been in the planning for six months, had been scheduled for last Friday in the Netherlands, but was postponed because of the Algerian hostage crisis.
The next election is due in 2015.
Meanwhile, an online poll in Le Figaro newspaper suggests many French people would be happy to see Britain leave.
With more than 15,500 votes cast, 70% favoured Britain leaving.