London (AFP) - Members of Britain's main opposition Labour Party launched a campaign Wednesday to leave the EU at an upcoming referendum, the latest initiative as the debate over Britain's EU membership heats up.

The Labour Leave campaign is fronted by four veteran MPs and backed by millionaire businessman John Mills, the party's biggest individual donor in recent years.

While Labour leader Jeremy Corbyn was anti-EU before being elected to the top job last year, he now says the party will campaign for Britain to stay in the EU in a referendum expected to be held this year.

Kate Hoey, one of the Labour MPs launching the Labour Leave campaign, called the EU "anti-democratic, anti-socialist, unaccountable".

"I think this is going to end up in a situation where we have the establishment actually leading on this and the people rebelling and that's what we'll be working for," she said.

Two main movements have already been established to campaign for Britain to leave the EU -- Vote Leave and Leave.EU.

But organisers expect all of them will work together closely as the referendum approaches.

Prime Minister David Cameron wants Britain to stay in the bloc assuming he can secure concessions on issues such as the level of benefit payments to EU migrants coming to Britain, although members of his party have said they will campaign against EU membership regardless.

Officials say it is possible he could agree a package of measures with European partners at a Brussels summit next month, which could then pave the way for a referendum in June.

Another anti-EU Labour MP, Graham Stringer, claimed 55 percent of Labour voters wanted a serious renegotiation of the terms of Britain's membership of the EU, adding Cameron had asked for "very little".

Mills said that most people in Britain wanted free trade with Europe but without being involved in the "political superstructure" there.

A Survation opinion poll for this week's Mail on Sunday newspaper indicated that 53 percent of people wanted Britain to leave the EU, compared to 47 in favour of remaining, without counting undecided voters.