Constitutional experts have argued that the Government was wrong to allow the couple their civil wedding in Windsor three and a half years ago.

The discovery of documents by Michael Jones, who is researching for a book about secret dealings between Buckingham Palace and Downing Street, has prompted allegations of a cover-up by the Government.

Downing Street has always refused to publish the legal advice that persuaded ministers to allow Charles and his second bride a civil ceremony.

The controversy centres on the Marriage Act of 1836, which introduced civil weddings for the first time, but specifically banned royals from them.