The Labour Party is threatening to discipline four of its senior MPs for backing a campaign calling for a referendum on the new European Union reform treaty, one of the legislators said on Monday.

The four MPs, including former ministers, are part of the advisory group of "I Want a Referendum.com".

The group is protesting against the government's refusal to hold a plebiscite on the Lisbon treaty by staging its own unofficial referendums in parts of Britain.

Labour promised in its 2005 election manifesto to hold a referendum on a proposed EU constitution which was later rejected by French and Dutch voters.

Prime Minister Gordon Brown argues the Lisbon treaty, agreed on by EU leaders last year, is much less far-reaching than the constitution and does not need to be put to a vote.

Labour leaders are furious about their MPs' support for the pro-referendum campaign because some of the mock referendums are being held in parliamentary seats held by Labour ministers.

Labour MPs discussed the issue at a closed-door meeting on Monday and a party leader referred it to a Labour parliamentary committee which could recommend disciplinary action against the four, a party spokesman said.

"We are in the absurd position where we may actually be punished for trying to maintain a manifesto pledge," Frank Field, one of the four Labour MPs, told Reuters.

He said he did not know what sanction the Labour committee might recommend but suggested it could be a suspension.

The other three Labour MPs are Kate Hoey, Graham Stringer and Gisela Stuart, a member of the convention that drafted the now-defunct EU constitution.

The Lisbon treaty, designed to update the EU's institutions for the new 27-member bloc, is being debated by the parliament, where Brown is expected to have a big enough majority to win ratification, even though the Conservative Party will demand a referendum.

"I Want a Referendum.com", which includes members of all the leading political parties, plans to send ballot papers to half a million people. The mail-in vote will be overseen by Electoral Reform Services, an independent ballot administrator.

Voters will be asked "Should the UK hold a national referendum on the EU's Lisbon treaty? Yes or no?" and "Should the UK adopt the EU's Lisbon treaty? Yes or no?"

Results will be announced before the end of February. The ballot will have no legal value but is intended to raise pressure on the government to call a referendum.