Former prime minister Sir John Major has said the surge of support for anti-Europe parties in the EU election will make it "much easier" for David Cameron to renegotiate Britain's position in the alliance.
He also insisted a referendum was necessary as a "reaffirmation" of the UK's membership and to "relieve bitterness" around the issue.
The ex-Tory leader, who negotiated the Social Chapter and euro opt-outs from the Maastricht Treaty, told the BBC Radio 4 Today programme: "The results of these elections right across Europe have made a renegotiation much easier.
"It is apparent now to governments right across Europe that reform of the EU is necessary, it isn't working as it should, it isn't working in the way which European citizens think it should.
"I think that gives a great deal of power to the British determination to renegotiate because they will have allies today which, in the 1990s, we frankly didn't have."
Asked about the EU's fundamental principle of free movement of people, he said it could not be restricted altogether but could not be left unqualified either.
"The difficulties we are facing are difficulties we are not going to be facing alone," he said. "That does begin to change the circumstances... that is an issue that will have to be addressed by governments other than us.
"I think there are things that can be done, as the Prime Minister has already said - freedom to take up work, not benefits. I don't think you can have an absolute restriction on movement but maybe you can qualify it in different ways and that is something I think would find an echo in many European governments as well as here."
He called a referendum "absolutely the right policy", adding: "There are many people in this country who weren't born when we had our last referendum when we entered the EU.
"Plainly the union has changed and changed dramatically from the old common market and trading bloc. I think there is a great deal of resentment among people who say it has changed into something I didn't think we were joining a long time ago.
"So I think it is necessary to have a reaffirmation of our membership in order to relieve this bitterness there has been in British politics about Europe for so long... I do believe the Prime Minister will win this referendum. There will be significant things that can be renegotiated."
He also argued that the principle of subsidiarity - the idea that decisions are taken as close to the citizen as possible - had been "wrecked" by the European Commission, leading to the frustrations made manifest in last week's election.
But he said the solution was not to leave the EU, which would mean losing free access to the single market.
He added: "We can't just walk out of Europe, we have inherited liabilities, we would have to negotiate our way out. It would cost billions.
"We would find ourselves very isolated and in a very difficult position without Europe. I know that is uncomfortable for people who are frustrated but the Government and the country has to realise in a cold, clear-eyed way what is in its own economic self-interest and what is in the self-interest of the future of the UK and our well-being in the future."
He dismissed Ukip as a contender long-term, saying its appeal is not one that is "instinctively likely to continue" for very long.
"They are there and of course they are an impediment for the moment. But what we are looking at is not a party political problem but the future of the UK and the well-being of this generation and the next," he added.
"At the moment David Cameron is the only party leader with any chance whatsoever of firstly delivering a referendum... and secondly delivering the changes in Europe that I believe are necessary and which I believe can be negotiated."
He gave his success at negotiating Britain out of the euro and Mr Cameron's at reducing the budget as examples of things that people said could not be achieved which were.