Party aims to step up pressure on Tories by backing in-out referendum on Europe as early as its autumn conference

Labour is considering backing an in-out referendum on Europe as early as its autumn party conference.

Sources say the shadow cabinet has been considering a range of options, including backing a "clear the air" referendum on Britain's membership to be staged either before the 2015 general election or six months after it.

The party is even looking at the option of tabling amendments to the upcoming private member's bill on an EU referendum in 2017, being promoted by the Conservative MP James Wharton.

The Labour amendment would propose a pre-2015 date for the referendum, which would place some Tory MPs in a dilemma over whether or not to stick with the Conservative leadership's position that no referendum should be held until the outcome of negotiations with EU partners on the terms of UK membership some time in 2017. A referendum could be staged on the same day as the European parliamentary elections on 22 May next year.

Wharton's bill is due to start its parliamentary process next Friday with its second reading. The foreign secretary, William Hague, is expected to speak and Cameron has imposed a three-line whip to push the bill through, but even Labour supporters of a referendum are likely to stay away due to the partisan way in which the Tories are promoting it.

A more mainstream option under consideration in the shadow cabinet is a manifesto commitment to hold a referendum on UK membership in the first six months after the general election, similar to the Labour pledge in 1997 to hold a referendum on Scottish devolution. Labour would support Britain remaining inside the EU.

The final option is to commit to a referendum on a permanent footing if there is any change in power for Brussels – a small tightening of the existing legal commitments to a referendum if there is a substantial transfer of powers to the EU. The in-out referendum has the support of senior figures including the shadow chancellor, Ed Balls, the shadow defence secretary, Jim Murphy, the party policy co-ordinator, Jon Cruddas, and the shadow health secretary, Andy Burnham, but the shadow foreign secretary, Douglas Alexander, is continuing to advise leader Ed Miliband to keep his options open and make no commitment at this stage.

He argues that the state of European politics is still in flux, including speculation that Germany will demand treaty changes to strengthen the supervision of the euro. The German chancellor, Angela Merkel, goes to the polls on 22 September, just as the Labour party conference convenes in Brighton

Senior Labour figures do not seem sympathetic to the idea of a referendum staged after a negotiation with the EU on the terms of British membership, broadly the proposal advanced by David Cameron in a speech in January. Although Labour supports reform of the EU, it is not seeking a full-scale renegotiation of the UK relationship including treaty changes.

Labour sources said it was unclear how the leader would resolve the debate but a decision may come in the early autumn, possibly at the party conference, leaving enough time to back an amendment to the Wharton bill.

It is also acknowledged that the Liberal Democrat leader Nick Clegg has already shifted his ground by saying the referendum is a matter of "when rather than if", so risking Labour being the only party not to support a referendum at the next election. Labour has said it would abstain on the Wharton bill next week.

A number of pro-referendum Labour MPs may stay away from Westminster due to their distaste about the partisan Tory approach to the referendum. Balls emphasised in February that Labour would be "stupid" to campaign in 2015 against a referendum, while Murphy has similarly hinted that one could be staged earlier. Last October, he said: "I think at some point there will have to be a referendum on the EU. I don't think it's for today or for the next year, but I think it should happen."

Cruddas has long supported a referendum. He said in 2011: "This is about democracy. This is about respecting the people … If we do not have a real referendum then anger and resentment will grow. We have to be bold and let the people into this conversation."

This approach would be welcomed by LFR (Labour for a Referendum), a group of Labour MPs which have campaigned for the leadership to back an EU vote since early May.

Last month, Miliband criticised the prime minister's promise on EU referendum, saying: "Our national interest lies in staying in the EU and working for the changes that will make it work better for Britain.

"It is wrong now to commit to an in-out referendum and have four years of uncertainty and a 'closed for business' sign above our country. Let me be very clear: we will always make decisions on these issues in the national interest."