When Gordon Brown invited Wendy Alexander, the Labour leader in Scotland, to reverse her oft-repeated antipathy to a referendum on Scottish independence, she appears not to have hesitated. Wendy, like her sibling, Douglas, the International Development Secretary, is among the Prime Minister's fawning loyalists. She knows that an early referendum is now the only route to the answer Mr Brown needs. The Conservative local election triumph has changed utterly the politics of unionism.

While Labour retains power, Scots might be persuaded to keep the Union and with it the phalanx of Scottish Labour MPs without whom only Tony Blair, among modern Labour leaders, won a parliamentary majority. A Tory government with a handful of Scottish seats would revive complaints about Scotland's democratic deficit, inflate support for the divorce that Alex Salmond craves and render impossible the election of another Scottish prime minister.

But if Ms Alexander's handbrake turn leads to a hasty vote on Britain's future, the real democratic deficit will be in England. The 1707 Treaty of Union, passed both by the Westminster and Edinburgh parliaments, reflected the combined will of both political establishments to augment the union of the crowns with a merger of national legislatures. Ending the Union demands the same mutual consent.

When Donald Dewar ignored England in the referendum that authorised Scottish devolution, senior Labour figures questioned his logic. They were mollified by his insistence that English interests were not harmed - Westminster remained sovereign and Holyrood a dependent subsidiary. This time, though the English must be consulted. True, they might cry good riddance to their irksome northern neighbours. But it is not madness to hope that wise majorities on both sides of the Border will assert themselves.

A referendum would allow the case for union to be explained with an enthusiasm that Mr Brown's dour rhetoric never conveys. Mr Cameron could help. The people of this nation state, who stood united against Nazism, are entitled to know what will be lost if Britain is obliterated.

The constitutional vandalism that Labour launched on the pretext that it would cement the Union has never been subjected to popular scrutiny. It must face that test before it provokes schism. That means a three-question referendum - independence, devolution or a unitary state - with all the British people eligible to vote. The electorate that is trusted to elect Mr Brown as Prime Minister is entitled to determine the future of the nation he purports to lead.

Tim Luckhurst is Professor of Journalism at the University of Kent and a former Editor of The Scotsman

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