Two senior Labour MPs have pledged to continue campaigning for an EU treaty referendum despite pressure from their party to stop.
I Want a Referendum was launched by a cross-party group of MPs
Contrary to earlier reports, Frank Field and Kate Hoey have both said they have not quit I Want a Referendum.
Two other Labour MPs, Gisele Stewart and Graham Stringer, have had their names removed from campaign literature but continue to support its aims.
The campaign is holding referendums in 10 Labour and Lib Dem constituencies.
One of the MPs in whose constituency a vote is being held, Andrew Slaughter, has accused the campaign of being a "dishonest" front for the Conservative Party aimed at unseating Labour MPs.
But the Labour-supporting head of its advisory board, Derek Scott has insisted it is a genuinely cross-party effort.
He said the reason Labour and Lib Dem MPs had been targeted was to remind those parties of their 2005 manifesto commitments to hold a referendum on the EU constitution, which opponents say is essentially the same as the treaty.
He added that the campaign was targeting "a mixture of big names, if you like, and less well-known people - and a combination of Labour and Liberal Democrat, as it is those two parties who have basically reneged on the promise they gave before the last election".
Mr Scott clashed with Mr Slaughter over the campaign's aims in an interview for BBC Radio 4's Today In Parliament.
Labour's Chief Whip Geoff Hoon is understood to have threatened the four Labour MPs on the I Want a Referendum board with expulsion from the Parliamentary Party.
Lib Dem call
But Mr Field, a former welfare minister, said he had resisted pressure from Labour's whips and would be keen to have one of the referendums in his Birkenhead constituency.
Vauxhall MP and former sports minister Kate Hoey said she would also continue to campaign for a referendum on the treaty.
Lib Dem home affairs spokesman Chris Huhne, whose Eastleigh constituency is one of those where a referendum will be held, challenged campaigners to put the wider question of whether the UK should be in the EU to the vote.
He said there was no "practical or financial impediment" to asking I Want a Referendum campaigners from putting such a question on the ballot.
"If they refuse, my constituents will be entitled to conclude that this so-called "cross-party campaign" is afraid to ask the real question and they will draw the conclusion that this is just a Tory front organisation," added Mr Huhne.
Prime Minister Gordon Brown has resisted calls for a referendum on the treaty, arguing it is substantially different from the constitution document and Britain has "red lines" protecting sovereignty in key areas.
MPs are currently debating the ratification of the treaty - but the Conservatives, who are the only party at Westminster demanding a referendum, would need to gain the support of Lib Dem and Labour MPs to get their way.
Hear the exchanges between Derek Scott and Andrew Slaughter on Today in Parliament on BBC Radio 4 at 2330 GMT on Friday, 8 February or download the podcast from the Radio 4 website.