The admission by the Department for Constitutional Affairs contradicts repeated assertions by Clarence House that she will be known only as Princess Consort on her husband’s succession, and not Queen.

Any change to her status may also have to be approved by the 17 parliaments in countries where the new King will be head of state. However, that runs the risk that some countries, such as Australia, Canada and New Zealand, may vote instead to become republics because they could not accept a “Queen Camilla”.

The Prince and Mrs Parker Bowles are to marry in Windsor on April 8.

Clarence House was still arguing last night that it was only a convention not a right that the wife of a King was automatically known as Queen. But while maintaining its legal advice was correct that she will be Princess Consort, it conceded that if there was a requirement for legislation to prevent her becoming Queen it would abide by it. However, any government Bill would not be enacted before the Prince had succeeded to the throne. He privately has always hoped that public opinion would shift in favour of a Queen Camilla.

A spokeswoman for the Department for Constitutional Affairs, headed by Lord Falconer of Thoroton, the Lord Chancellor, said that an Act of Parliament would be required.

“I think traditionally that’s probably the case because in all similar circumstances in the past royal marriages that is what has happened,” she said. “But I think she is not going to be referred to as Queen, she will be referred to as the Princess Consort.”

Asked about the position of other countries where the Prince of Wales would become head of state on his succession, the spokeswoman replied: “I think you are right in thinking it would require legislation for her not to be Queen.”

Another official at the department said: “There definitely has to be legislation because it is changing the role of the wife of the monarch. That has to be approved by Parliament.”

Downing Street was brought into the controversy yesterday when Tony Blair’s official spokesman said: “The position at the moment is limited to what the title would be on her marriage. In terms of any future events, let’s wait until future events arise.”

On the question of whether Mrs Parker Bowles would automatically become Queen in the absence of legislation, the spokesman added: “I’m not disputing what you have said.”

The revelation will destroy Clarence House’s carefully crafted strategy that it was “intended that” Mrs Parker Bowles would be known as the Princess Consort on succession.

Officials privately admitted that they had deliberately used the phrase “intended that” to leave the option open for a Queen Camilla if public opinion shifted in her favour.

A Clarence House spokesman challenged the Government’s interpretation of the law last night. “Our position is clear. We regard it as an issue of convention not statute. It has never been written in statute that the King’s wife is automatically a Queen but there are always different interpretations of the law,” the spokesman said.

But the official conceded that if the Parliament of the day insisted on legislation to prevent her becoming Queen they would follow the advice of ministers.

“There is the Civil List Act,” he said, adding that the Act tidied up and refreshed royal protocol at the beginning of each new reign. “We would use that to clarify the position.”

Christopher Leslie, the Constitutional Affairs Minister, confirmed the status of the marriage yesterday in a written parliamentary answer to the Labour backbencher Andrew Mackinlay.

The Thurrock MP asked whether the proposed marriage of HRH the Prince of Wales to Camilla Parker Bowles was “morganatic”, by which Mrs Parker Bowles would not inherit on the Prince’s death.

Mr Leslie replied: “No.”

Mr Mackinlay said: “This is a mess of the Royal Family’s own making.”

Patrick Cracroft-Brennan, the editor of Cracroft’s Peerage, said: “It has been a tradition for more than 1,000 years that the wife of the King automatically becomes Queen. They will have to legislate. If they don’t they are tampering with centuries of English tradition and history.”

The title Princess Consort was an invention not previously known in British constitutional history.

It was designed to reflect the fact that a large section of public opinion is unwilling at present to accept a Queen Camilla.

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