Lord Tebbit called on David Cameron yesterday to pledge a referendum on the new EU reform treaty even if it has already been introduced.

The Tory leader is facing increasing pressure to spell out what he will do if, as expected, the Irish vote to ratify the Lisbon treaty just three days before the start of the Conservatives’ annual conference.

A Yes vote next month makes it likely that the treaty will be in force by the general election. Unless Mr Cameron provides activists with a “satisfactory assurance” that he would hold a referendum regardless, he risks losing ground to UKIP, Lord Tebbit said.

The former party chairman said that Mr Cameron should include a commitment to a retrospective referendum in the Conservatives’ next manifesto.

He added: “It’s perfectly clear that when David Cameron originally gave his undertaking it was nothing about the Irish vote at all. It was an undertaking that there would be a referendum. End of message. Full stop. And I think the party would expect that he would stick to that undertaking.

“My understanding is that they’ve reneged on that commitment and if the Irish vote Yes and the treaty is ratified throughout the community Mr Cameron’s only commitment is that he will not let the matter rest there. I don’t know what that means.”

The peer, who has come close to expulsion in recent months for his outspoken remarks on Europe, called on Mr Cameron to demand wholesale changes in the EU.

“Mr Cameron must not forget that things which are set out in the election manifesto, if he wins the election, would not be obstructed in the House of Lords. If he comes up with something after the election which was not in the election manifesto, it is quite possible that the House of Lords might turn it down. That is convention.”

Mr Cameron’s senior colleagues concede that the Irish vote poses a “headache” for him before the party’s annual conference in Manchester. He is being urged by some to tackle the issue head-on.

However, a senior Conservative said that Mr Cameron regarded Afghanistan as his most important foreign policy priority and wished to avoid a long-running battle with Brussels.

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