The UK’s Labour party has left open the possibility of agreeing to an in-out referendum on Britain’s EU membership as foreign secretary (minister for foreign affairs) William Hague mocked Labour party leader Ed Miliband for “confusion and weak leadership”.

As Tory (Conservative) MPs packed the House of Commons (the UK's lower elected house) to debate an EU referendum bill, the shadow foreign secretary, Douglas Alexander, dismissed prime minister David Cameron's “arbitrary” plan to hold a vote in 2017.But Alexander declined to rule out a referendum definitively as he said Labour would be governed by the national interest.

The foreign secretary launched a withering attack on the Labour party as he quoted from a series of frontbenchers expressing varying opinions on whether to hold a referendum.

With most Labour MPs boycotting the session, Hague said: “With some in favour, some against, some adamantly for not having one, some adamantly for deciding later at some point, perhaps before the general election but who knows - with such a shambles of confusion and weak leadership, no wonder [Labour MPs] are wondering what they are here for and where their leader is now. One day Unite [the British trade union] will give them the orders of how to vote.”

The exchanges between Hague and Alexander dominated the proceedings as MPs held their first substantive debate on a private member's bill, tabled by the Tory MP for Stockton South, in north-east England, James Wharton, that would pave the way for an in-out referendum on Britain's EU membership by 2017.

David Cameron fully supports the Wharton bill which, in the absence of Liberal Democrat support for a government measure, is the only way in which the Tories can vote in favour of the prime minister's 2017 referendum pledge.

The Labour leadership, which had planned to ignore the Wharton bill, is embarking on an intense debate on whether to embarrass the government by tabling an amendment to call for a referendum before the 2015 general election. Some senior figures warn this would be irresponsible because it would jeopardise Britain's EU membership. A decision will have to be made by November when the bill will be considered at committee stage.

Alexander chose his words carefully as he left open the possibility of agreeing to a referendum. Asked by the Tory MP Geoffrey Clifton-Brown whether he could give an “absolute” assurance that Labour would not change its mind in opposing a referendum before the 2015 general election, the shadow foreign secretary said: “We have maintained our position. Any judgment in relation to an in-out referendum has to be based on the national interest. Our judgment is that the national interest is not served by this bill and that is why we do not support it.”

Alexander, who mocked the prime minister for changing his position after saying in 2006 that the Conservative party should stop “banging on” about Europe, said that a referendum on Cameron's timetable would not be in the national interest.

“Any judgment about an in-out referendum on the UK's membership of the EU has to be based on a judgment of what is the national interest... [This] bill reflects an arbitrary date unrelated to the likely timetable of major treaty change.

It represents an unrealistic and uncertain negotiating strategy and it is brought forward by a party divided between those seeking consent and those seeking exit.”

Alexander said Tory MPs were forcing the issue through a private member's bill because they no longer trusted Cameron. Looking across the chamber at the prime minister, Alexander said: “Why doesn't the Conservative party trust the Conservative prime minister? When are they going to release the Downing Street One? He is sitting there like a hostage on the frontbench [where government ministers sit], not like a leader.”

But the shadow foreign secretary said Labour supported the principle of a referendum because it supported the European Union Act of 2011 which would trigger such a vote if major powers were passed from the UK to the EU.

A series of Labour MPs, including the former ministers Frank Field and Keith Vaz, said they would vote in favour of the referendum bill. Dennis Skinner, the veteran Eurosceptic Labour MP, intervened in Wharton's speech to say: “In 1971 I voted against the then common market and I voted for a referendum in 1975. Sadly the country didn't follow my advice or we wouldn't be doing what we're doing here today.

However, there is one thing deficient in this bill. It doesn't ask for a referendum until 2017. What we need is a referendum before the next election. Will he give that commitment?”

Wharton replied: “For the first I find myself at least agreeing with part of [your] comments. I do wish more people had listened to him in 1975. But I'm sorry they're not going to now.”

But Wharton said that amending his bill would wreck the measure. “Parliamentary procedure is one of the greatest challenges this bill will face. Were it drawn, enlarged and [made into a] complex bill it would be easier to slow its progress and wreck the chances of it getting through and delivering what the British people want.”

By arrangement with the Guardian