A final attempt by MPs to force a referendum on Europe will be made tomorrow when the Bill implementing the Lisbon Treaty reaches its crucial stage in the Commons.
The Liberal Democrats, who hold the key to the vote, came under pressure to back a proposal for a "double referendum" on both the Lisbon Treaty and whether Britain should remain a member of the EU.
An amendment to the Bill paving the way for a two-question referendum has been tabled by the Labour MP Ian Davidson and could unite about 30 rebel Labour MPs, most Tories and some Liberal Democrats – enough to defeat the Government. The Liberal Democrats want an "in or out" referendum on EU membership while the Tories want one on the treaty.
Tomorrow's crunch vote could turn into the first trial of strength between Nick Clegg, the Liberal Democrat leader, and his own MPs. He has ordered them to abstain on the Tory proposal but up to 15, including some frontbenchers, may defy him by supporting a referendum on the treaty. David Heath, the constitutional affairs spokesman, and Alistair Carmichael, the Scotland spokesman, are expected to back a treaty referendum.
Mr Clegg made clear yesterday that his MPs would be on a three-line whip to abstain on the Tory proposal. But he held back from threatening frontbenchers with the sack if they voted against the party line. He said: "Anyone who has been in those discussions knows that this is a collective discussion and if they were not to follow that three-line whip that would then to lead to developments which would arise between myself, those in question and the chief whip."
Mr Clegg published an Ipsos MORI poll showing that 58 per cent of people backed his call for a full referendum on membership of the EU, twice the number in favour of a treaty referendum. But the Eurosceptic I Want a Referendum campaign released an ICM survey of 1,000 people who voted Liberal Democrat at the last election, showing that 70 per cent want the party to back a "double referendum".
The Tories made a last-minute plea to the Liberal Democrats to honour their 2005 manifesto pledge to back a referendum on the EU constitution, the treaty's forerunner. They said the Government could be defeated if Mr Clegg's party joined them in voting for a public vote. "The LibDems have an absolutely pivotal position," said William Hague, the shadow Foreign Secretary.
Two men scaled a crane near Parliament yesterday and unfurled banners demanding a referendum on the treaty. Jim Murphy, the Europe minister, said: "The place to make these decisions is in this chamber – not on a crane halfway above the city sky of London."