A “referendum lock” on future transfers of power from London to Brussels was published by the Foreign Office on Thursday, amid warnings from strident Conservative Eurosceptics that the measure does not go far enough.

David Lidington, Europe minister, said new legislation would give “the British people the key” in deciding whether the UK should ever agree to deeper European integration, in areas such as foreign policy, tax, border controls or the euro.

But the “flagship bill” does not satisfy the demands of many Tory MPs who do not simply want to stop development of the European project: they want David Cameron to repatriate existing powers.

“We are planning our next move,” said one of the 37 Tory MPs who defied government whips on a European motion last month. “There has never been a better moment to push for this.”

The European Union bill, published on Thursday, gives the public a veto on any treaty change that involves a transfer of national power or a policy area into the field of European decision-making.

It also provides for a referendum on any proposal to give up Britain’s veto in areas not requiring a treaty change, such as foreign policy and on agreeing the seven-year EU budget.

Proposals to join the euro, on deeper defence co-operation or opting in to the Schengen EU border accord are also covered by the measure.

But Douglas Carswell, Tory MP for Clacton, argues the bill is not legally watertight and could still allow ministers to cede powers to Brussels without a referendum. He says the coalition has shown its willingness to do so, in areas including the European arrest warrant and signing up to regulation of the City, including hedge funds.

All opinion polls suggest the British public would wield their veto if asked to approve a shift in power to Brussels, so the bill in effect halts any new EU treaty covering all 27 member states.

However, the bill specifically does not halt the proposed treaty amendment – requested by Germany – to put a eurozone bail-out facility on a permanent footing to avoid another Greek-style sovereign debt crisis.

Mr Lidington argues that the proposed change affects only the eurozone, but some Tory MPs insist it would impinge on the UK through increased Brussels surveillance of Britain’s budgetary plans.

The bill is part of a coalition compromise. Nick Clegg, the Liberal Democrat leader, agreed to the text on the understanding that Mr Cameron would not try to reclaim existing powers from Brussels.

“We are not going to reopen this issue of the repatriation of powers,” Mr Clegg told the Financial Times this month.

But eurosceptics believe Mr Cameron has two golden opportunities to reclaim powers for Westminster, including on working hours.

Mr Carswell argues the prime minister should make such a claim, for approving the German-sponsored treaty change to improve eurozone governance. The eurosceptics also view the forthcoming EU budget negotiation as another chance to deploy a veto.

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