Hundreds of Tory MPs will attend Commons for second reading of James Wharton's bill to force in-out vote by 2017

Conservative MPs start the long process of securing an in-out referendum on the EU by 2017 on Friday, amid growing signs Labour may try to trump the Tories by backing a similar vote before the general election.

Hundreds of Tory MPs will turn up at Westminster to vote for the second reading of a bill to introduce a referendum by 2017, which is sponsored by the Conservative MP James Wharton.

The Guardian disclosed last week that Labour was considering an amendment to the bill to propose a pre-2015 date for the referendum. The shadow cabinet is currently discussing the possibility, but Ian Austin, the shadow transport minister and a close ally of senior shadow cabinet members, has already used an newspaper article to back a pre-election date.

If Labour does decide to go ahead with amendment, it will not table it until the Commons report stage in late autumn. As such, the bill should pass its second reading relatively easily.

A Labour amendment would pose a dilemma for Tory MPs, many of whom support an early referendum but have been willing so far to co-operate with David Cameron's strategy of trying to negotiate treaty changes, and to stage a referendum by 2017 if they are not made.

Influential figures such as Liam Fox may speak in Friday's debate , which would mark a rare intervention since he resigned as defence secretary. Fox, one of 12 sponsors of the Wharton bill, has said he has no fears about life outside the EU. He would be one of many MPs who would have to think seriously if Labour tabled their amendment.

Wharton himself has said that any amendments are likely to kill the bill as it needs to be changed as little as possible if it is to reach the statute book as a private members bill. Any move by Labour to force an earlier date for a referendum would also cause Cameron to think about how he could kill off the bill.

The shadow cabinet is deeply divided on how to proceed. Ed Miliband has insisted there is no need to make a decision on a referendum now, reasoning that discussion of such a possibility could jeopardise the country's economic recovery and put potential investors off. That view is strongly shared by the shadow foreign secretary, Douglas Alexander.

Austin, a long time ally of the shadow chancellor Ed Balls, writes in the Express and Star: "The truth is that the UK needs to decide and I would prefer to do so more quickly. I know this is not Labour party policy but my view is that we should have a referendum next year on the same day as the European elections."

He adds that he would personally keep the free trade single market, but thinks that "we should decide social legislation for ourselves". Austin insists he is setting out his own views and not acting on anyone else's behalf.

Number 10 responded to the speculation about Labour by saying the PM's views on a date are absolutely unchanged and that he would urge people to support the position he had set out.

The Downing Street spokesman added: "The right thing to do is to seek a mandate for that renegotiation and then for the referendum. That's what's going to be in the Conservative party manifesto."

He said Cameron was optimistic about attracting support to for his position.

In a bid to cheer up his MPs, Cameron was hosting an eve of battle barbecue in the gardens of Number 10 as his backbenchers stayed in London for the vote.