UK PM David Cameron
David Cameron is to promise an in/out referendum on the EU if the Conservatives win the next election.
The Conservative leader wants to renegotiate the UK's relationship with the EU, before asking people to vote.
Once a new settlement has been achieved, the British people can vote either to accept it, or to leave the EU, Cameron will say.
Labour's Ed Miliband said the speech showed the PM was "weak" and "driven by his party", not the national interest, reports the BBC.
The referendum is thought likely to take place during the early part of the next parliament if the Conservatives win the election.
The speech had been scheduled for last Friday in the Netherlands, but was postponed because of the Algerian hostage crisis.
The Conservative leader has been under pressure from many of his MPs to give a binding commitment to a vote on Europe.
Setting out the conditions for a future referendum, Cameron will say: "The next Conservative manifesto in 2015 will ask for a mandate from the British people for a Conservative government to negotiate a new settlement with our European partners in the next parliament.
"And when we have negotiated that new settlement, we will give the British people a referendum with a very simple in or out choice to stay in the EU on these new terms; or come out altogether. It will be an in-out referendum.
But he will say that holding an in/out referendum now would be a "false choice" because Europe is set to change following the eurozone crisis.
The speech, taking place in central London, is expected to be watched closely by other European leaders, the business community and supporters and critics within his own party.
Several Conservative MPs - who want a looser relationship with the EU focused around trade and who have been briefed about the speech - say they are "satisfied" with the thrust of what Cameron is going to say.
But some europhile Conservatives, including Lord Heseltine, have warned that committing to a referendum at some point in the future on the outcome of an uncertain negotiating process is an "unnecessary gamble".
'Years of uncertainty'
The Lib Dems say pursuing a wholesale renegotiation of the UK's membership will cause uncertainty and deter foreign investment while Labour claim Cameron's approach is being driven by party calculations rather than the national interest.
Labour leader Ed Miliband said the speech would define Cameron "as a weak Prime Minister, being driven by his party, not by the national economic interest".
"In October 2011, he opposed committing to an in/out referendum because of the uncertainty it would create for the country. The only thing that has changed since then is he has lost control of his party and is too weak to do what is right for the country," he said.
"Everyone knows that the priority for Britain is the jobs and growth that we need. We have had warning after warning from British business about the dangers of creating years of uncertainty for Britain.
"Britain needs a prime minister who is making change happen now in Europe, ensuring that we put jobs and growth ahead of austerity and unemployment."