Royal daughters could be given an equal claim to the throne and the ban on heirs marrying Roman Catholics may be scrapped under plans discussed by Downing Street and Buckingham Palace.
The issue is due to be raised in the House of Commons today during a debate on legislation, introduced by Liberal Democrat MP Evan Harris, to end the "uniquely discriminatory" rules laid down by the 1701 Act of Settlement. Dr Harris has cross-party support, but is unlikely to win the backing of ministers at this stage. The Government has said changes would be a "complex undertaking".
The Prime Minister Gordon Brown is said to believe that agreement should be reached between Commonwealth countries before he gives the bill his formal support, and he said he would raise the issue at November's Commonwealth Heads of Government conference. Mr Brown was reported to have called the 1701 Act "an anomaly that has no place in the 21st century". There would be no change, however, to the requirement that the monarch be a Protestant.
Dr Harris' Royal Marriages and Succession to the Crown (Prevention of Discrimination) Bill is co-sponsored by Catholic parliamentarians including Tory MP Edward Leigh, Labour's Andrew Mackinlay and John Grogan, and Liberal Democrat John Pugh.
Dr Harris said: "This Bill will remove the uniquely discriminatory rule which currently exists – that an individual in the line of succession to the throne can have a civil partnership with a Catholic, can marry a Muslim or atheist, but cannot marry a Catholic.
"It will also end the outdated rule which allows a woman in the line of succession to the throne to be automatically superseded by a younger male sibling. If our current monarch had a younger brother, we would never have had a Queen Elizabeth II."
A Ministry of Justice spokeswoman said: "The Government has always stood firmly against discrimination in all its forms, including against Roman Catholics, and we will continue to do so.
"To bring about changes to the law on succession would be a complex undertaking involving amendment or repeal of a number of items of related legislation, as well as requiring the consent of legislatures of member nations of the Commonwealth.
"We are examining this complex area although there are no immediate plans to legislate."
The Act of Settlement, laying down that heirs would lose their right to the throne if they married a Catholic or a convert, was passed to ensure that the Catholic Stuart dynasty could not return to the throne following the Glorious Revolution of 1688.
Last year Autumn Kelly renounced her Catholic faith in order to marry the Princess Royal's son, Peter Phillips, without damaging his position as 11th in line to the throne.
The Act also put into law the principle of primogeniture – the precedence of male heirs over female. If changes to that practice were made retrospectivly, Princess Anne, currently 10th in line to the throne, would leapfrog her two younger brothers, the Duke of York and the Earl of Wessex and their children to become fourth in line, behind Prince Harry.