A senior Tory has warned that David Cameron's leadership would be at stake if the Conservative party loses next year's European parliamentary elections, something most polls, political betting sites and the Ukip have said was likely to happen.

The warning came as the former deputy prime minister John Prescott became the most senior Labour figure to call on Ed Miliband to back an in-out referendum on Europe now, ahead of Cameron's planned date of 2017.

The warning that the European elections could prove to be a flashpoint for Cameron's premiership was made by David Ruffley, a member of the home affairs select committee and a former Tory whip. He said MPs in marginal seats would be unsettled if the prime minister had failed to turn around the party's fortunes at the ballot box next year.

Ruffley told Sky's Murnaghan programme: "I think next May's Euro elections might put pressure on him to go harder because there is a lot of speculation in and around Downing Street, so I am led to believe, that Ukip might come first.

"Now if that happens next May there'll be 12 months before the election and some of our colleagues in marginal seats might get a bit windy. I don't think Ukip are going to win seats but they could split the Conservative vote if they are strong and let Labour through in those marginal seats."

Ruffley said the Conservative party would give him time to see if his EU referendum bill reached the statute book.

A ComRes poll in January put Labour on 35% (up 19 percentage points on its performance at the 2009 European elections), Ukip on 23% (up six) Conservatives on 22% (down six) and Liberal Democrats on 8% (down five).

The Ukip leader, Nigel Farage, has already promised a political earthquake if his party tops the poll next year. The regional list electoral system used in the European elections is suited to Ukip because the party matters more than the individuals who get elected. Ukip is currently favourite to win.

Prescott intervened in the European debate, writing in the Mirror over the weekend: "Cameron thought he was shooting Ukip's fox by offering a referendum. All they want to do is save their seats in 2015. So Labour should call his bluff. We should continue to put the economy, jobs and the national interest first.

"That's why Ed Miliband should call for an in-out referendum on Europe. Not to be held in 2017 but NOW!

"Labour could still continue to argue the need for reforming Europe during the debate. Like the 1975 referendum, this would be a consultative poll, with parliament making the final decision. A public vote either way could strengthen our negotiation position for reform with our European partners.

"I believe during a referendum campaign, after both sides have been put, more people will decide to stay, just as they did in 1975."