Nick Clegg, the Liberal Democrat leader, may be about to make parliamentary history by ordering his colleagues — on pain of political torture, or worse — not to vote at all.

Next week, after months of legal argument and political squabbling, Parliament will vote on whether the Lisbon treaty should be put to a referendum. However, Lib Dems have been told to stay away from both the Aye and No lobbies, with members of the front bench being threatened with dismissal if they do not comply.

One disgruntled party MP told The Times: “It must be the first time in history that there has been a three-line whip telling MPs to abstain.”

Despite the unusual vigour with which Liberal Democrats are being ordered to sit on the fence, Mr Clegg still faces his first serious Commons rebellion, with party insiders predicting that up to 15 MPs may break ranks and support a referendum. Several of the party’s frontbench spokesmen are among the potential rebels, with three of the firmest supporters of a referendum — David Heath, Alistair Carmichael and Tim Farron — sitting in the Lib Dem Shadow Cabinet.

There is scepticism within the party over whether Mr Clegg will risk a confrontation, given the party’s manifesto commitment at the 2005 general election to put Europe’s previous constitution to a popular vote.

Another Lib Dem asked: “How can you be sacked for voting in favour of your party’s election manifesto?”

The question of how Britain’s most fervently pro-European party got itself into such a tangle may puzzle outsiders; supporters of Mr Clegg say that the blame lies not with him but with Charles Kennedy, his predecessor-but-one. It was Mr Kennedy who allowed himself to be pushed into agreeing to support a referendum on the old European constitution, giving the Lib Dems a manifesto commitment identical to that held by Labour.

Sir Menzies Campbell, his successor, later claimed that the new Lisbon treaty was sufficiently different to make a referendum unnecessary. He called for a wider referendum on Britain’s membership of the EU, an issue on which the Lib Dems were prevented from forcing a Commons vote on Tuesday.

The Deputy Speaker’s decision not to call the Lib Dem amendment prompted protests that led to Ed Davey, the party’s foreign affairs spokesman, being barred from the Commons for the day.

Mr Clegg’s spokeswoman confirmed that Lib Dem MPs were on a three-line whip to abstain. “It is important in the sense that we are trying to make a point about what we feel the referendum should be about,” she said. “If people don’t follow the party line they are pretty well aware of what the consequences could be.”

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