Charles' 'quiet campaign' to have Camilla crowned Queen

According to insiders, he wants to gain public support so that both can be crowned at Westminster Abbey.

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The quiet determination will alarm critics who still feel that because of the once-adulterous nature of the relationship, the duchess should never become queen.

Since their marriage was announced in February 2005, Clarence House has said that Camilla will be known as the Princess Consort when the time comes for her husband to take over from his mother.

But constitutional experts have long pointed out that, under present legislation, she automatically receives the title queen when her husband becomes king.

It would almost certainly require an Act of Parliament for her not to be queen.

According to the documentary – entitled Queen Camilla – the prince hopes that he can secure enough public support to ensure that such a move is not necessary.

It is not the first time it has been suggested that Prince Charles has a silent campaign in mind.

After their wedding, broadcaster Jonathan Dimbleby, the prince's biographer, said Camilla would definitely become queen.

He also criticised Clarence House officials who were accused of fudging the issue in the run-up to the register office ceremony in order to defuse public hostility.

"It might have been better if Clarence House had publicly said, formally, we agree that she will be queen but, actually, she prefers to be called Princess Consort," he said.

"A bit more clarity would have been better."

The documentary will be shown on Channel 4 on May 31, and claims it will reveal the "real" relationship between the prince and duchess.

In it, former royal staff disclose the lengths to which the pair would go to keep their relationship secret.

It also contains interviews with friends, including Lords Carrington and Gowries, both former senior politicians, and writer and broadcaster Angela Huth.

There has been a steady change in the public perception of Camilla.

By the time they married, an opinion poll indicated that 65 per cent of people thought they should tie the knot, but only seven per cent wanted Camilla to be queen.

Last year 57 per cent of people said she should not become queen.

A Clarence House spokesman said she could not comment on the documentary as she had not seen it, adding: "We said at the time they got married that the Duchess of Cornwall would be known as the Princess Consort and that position has not changed."