A Liberal Democrat MP has said he will defy his party's leadership - and risk his frontbench job - by voting for a referendum on the EU treaty.
David Heath says his views on Europe are well-known
Justice spokesman David Heath said he had told the party's chief whip he planned to vote against the party line.
It is now up to leader Nick Clegg to decide his future, he added.
Mr Clegg will order his MPs to vote against having a referendum on the treaty or to abstain - depending on the wording of a Conservative amendment.
The Lib Dems plan to put down their own amendment calling for a referendum on the wider question of whether Britain should remain in the EU.
Mr Clegg has said he agrees with the government that the EU treaty is "a rather different beast" to the EU constitution - on which all main parties had promised a vote - and therefore a referendum on it is not necessary.
Mr Heath - who was among the MPs who campaigned for a referendum on the EU constitution in 2004 - said he would also vote in favour of his party's planned amendment on the "in or out" question.
He said he had been "consistent over many years" in his view that there "had to be a referendum on Europe".
He stressed that he did not want to lose his frontbench job over the issue but he was prepared for that to happen "if that is the consequence of voting for a referendum on the treaty" and he acknowledged his future was in the balance.
"It has not yet been determined. The chief whip knows what my position is. We have been discussing it for some time," he told BBC News.
Mr Heath also attacked the I Want A Referendum campaign, which is holding a private referendum on the EU treaty in his Somerton and Frome constituency.
I Want a Referendum is organising private polls in 10 Labour and Lib Dem marginal seats in order, it says, to put pressure on those parties to fulfil their 2005 manifesto commitments on a referendum.
"They are campaigning in my constituency when they know perfectly well I am in favour of a referendum," said Mr Heath.
A small number of other Lib Dem MPs are thought to share Mr Heath's position on the EU treaty but the party's defence spokesman Nick Harvey emphatically denied media reports that he was one of them.
He said he would vote against a referendum on the EU treaty but for a referendum on the "in or out" question, in line with the party leadership.
He claimed this was in keeping with the Lib Dems' 2005 election manifesto, which he said promised to give voters a say on all of the EU treaties passed to date, not just the constitution.
The Conservatives are the only one of the three main parties at Westminster calling for a referendum on the EU treaty but they will need the support of Lib Dem and rebel Labour MPs to overturn Labour's majority of 66.
The SNP, Plaid Cymru, DUP and OUP are also expected to vote for a referendum on the EU treaty.
Nineteen Labour MPs rebelled against the government in the first ratification vote in the Commons last month - but more are expected to defy their party's leadership when the Commons votes on the referendum issue.
Two former ministers, Kate Hoey and Frank Field, have also vowed to continue campaigning for private referendums in 10 marginal Labour and Lib Dem seats - despite pressure from Labour's chief whip Geoff Hoon to quit the I Want a Referendum campaign.
MPs will resume debating the ratification of the EU reform treaty on Monday, when they will examine foreign policy, security and defence issues.
There will be a further eight days of Commons debate before the ratification bill goes to the House of Lords.
Labour veteran Tony Benn has written to all MPs urging them to vote for a referendum on the EU's Lisbon Treaty.
The former Cabinet minister, who campaigned against Common Market membership in the 1975 referendum, warned the Treaty would transfer "important powers" from Westminster to Brussels.
And he questioned whether Parliament would be freely giving its assent to the Treaty, signed by Prime Minister Gordon Brown in December, if Labour MPs were forced by a three-line whip to vote for ratification.