The I Want a Referendum group's move comes a week before MPs vote on a Conservative amendment to the treaty bill, calling for such a referendum.

Ministers say their 2005 promise of a vote on the EU constitution does not apply now as the treaty is different.

But the Tories, some Labour and Lib Dem MPs and the UK Independence Party say it is just a re-named constitution.

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I Want A Referendum has been holding its own local referendums in some marginal Labour and Lib Dem constituencies.

Some Labour MPs, including former ministers Frank Field and Kate Hoey, are supporting the group.

The Liberal Democrats leadership says it will not back the Conservative amendment and has instead called for a referendum on the wider question of Britain's membership of the EU.

We need to know when we will debate this issue. It's a debate the British people want Ed Davey, Lib Dems Lib Dem ejected from Commons What's in the treaty?

But on Tuesday, the Speaker did not call that topic for a debate, prompting an angry protest by Lib Dem foreign affairs spokesman Edward Davey, who was eventually ordered out of the Commons.

Neil O'Brien, director of the I Want a Referendum campaign, accused the Lib Dems of having "chickened out" of calling for a referendum on the treaty.

At the last general election in 2005 all three main parties promised a referendum on the planned European Constitution.

But the document was discarded later that year when it was rejected by Dutch and French voters.

The EU Reform Treaty, signed by Prime Minister Gordon Brown and other national leaders in Lisbon in December, was designed to replace the constitution.

Ministers argue that no referendum is needed as the treaty merely amends the EU's existing constitution, rather than overwriting it - as the failed constitution would have done.

But the Tories, UKIP and various groups, including I want a Referendum, say the two documents are substantially the same and that the public must have a say.

I Want a Referendum's lobbying of MPs is expected to last several hours.