Boris Johnson has insisted that British voters are entitled to a referendum on the European Union treaty, even if it has already been ratified by the time the Tories win power.

In an intervention that will anger David Cameron and risks plunging the Conservatives into another row over Europe, the London mayor said the public deserves to have a say on the constitution.

The treaty was backed by 67.1% of voters in Ireland’s second referendum when the results were announced yesterday. Only two countries, the Czech Republic and Poland, have yet to sign up. Brussels hopes they will fall into line quickly.

Tony Blair is the strong favourite to become the first president of the 27-nation superstate created under the constitution.

The Tory leadership has repeatedly refused to promise a referendum if the treaty is in force by the time of the general election. But Johnson said voters would be “very jealous” of the Irish if they were denied a referendum and insisted that a vote must be held irrespective of whether the treaty has been rubber stamped.

“I think the British people deserve a say on it. I think it would be right for such a debate to be held,” he said. The Lisbon treaty was ratified by parliament after Labour decided not to hold a referendum.

Johnson’s comments, on the eve of the Tory’s annual conference, expose a rift over Lisbon. Cameron is desperate to avoid a return to the party warfare over Europe.

Last night he tried to shut down the debate by issuing an e-mail to all party members insisting there would be “no change” in party policy and “no new announcements” on Europe at the conference, following the victory for the “yes” campaign in Ireland.

He insisted he would not be pressurised into saying what the party will do if the treaty is in force in all member states, claiming it could have only “one policy at a time”.

In an interview with The Sunday Times, Johnson said: “If we are faced with the prospect of Tony Blair suddenly emerging, suddenly pupating into an intergalactic spokesman for Europe, then I think the British people deserve a say on it ... I do think it would be right for such a debate to be held, particularly if the upshot of the Lisbon treaty is going to produce President Blair.”

The treaty creates sweeping EU powers over asylum, health, justice and social security. It scraps national vetoes in 61 areas, weakens Britain’s power to block legislation and could pave the way for a European defence policy and EU army. If the treaty comes into force the EU will have a network of embassies around the world.

Lisbon will create an EU president, an EU foreign minister and EU diplomatic service. The British government is backing Blair for the top job, which is expected to command an annual salary of at least £247,000.

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