The overhaul of the royal rules of succession to end discrimination against women and Roman Catholics has been backed by David Cameron as well as Gordon Brown.
The likelihood of reform moved closer yesterday when the Conservative leader welcomed the opening of talks between Downing Street and Buckingham Palace on amending the 308-year-old legislation. Mr Brown will raise the issue with Commonwealth leaders in the autumn with a view to winning their approval for the historic change.
Mr Cameron said yesterday he would like to see the rules of succession rewritten, although he stressed that any changes should not be rushed.
He said: "It does not make sense in the 21st century to say that men have priority over women when it comes to inheriting the throne. It does not make sense to say that the king cannot marry a Catholic. So we do need change but we have to recognise that the Queen is not just our queen. She is also the Queen of all the Commonwealth countries that have her as their head of state, so this is not an easy change to make. There is a lot of talking and listening that has to be done first."
The support of all major parties for rewriting the 1701 Act of Settlement has pushed the subject up the political agenda, although they agree reform is likely to take years to complete.
The Act states that a monarch, as the head of the Church of England, loses their right to the throne if they marry a Catholic or convert to the faith.
It also spells out the principle of "primogeniture", which gives male heirs precedence over daughters in succeeding to the crown. The issue would be spotlighted if Prince William becomes a father and his eldest child is a girl.
During Mr Brown's visit to South America he disclosed that he had discussed the subject with the Queen. He said: "I think in the 21st century people do expect discrimination to be removed and they do expect us to be looking at all these issues."
Downing Street said last night that there was no question of the succession laws being relaxed to allow a Catholic to sit on the throne. The aim was to end the discrimination against Catholic husbands or wives of monarchs.
The Government, however, blocked a backbench attempt to reform the succession rules yesterday. Jack Straw, the Justice Secretary, told MPs that a Bill, which was introduced by the Liberal Democrat MP Evan Harris, was not the "appropriate vehicle" for a change of this magnitude.
Mr Straw said the reform had become a "higher priority" for ministers, but he refused to be drawn on a timetable for government legislation to bring about the change. He did, however, say that it was important that Britain's constitution was brought up to date.
Dr Harris told the Commons that most people would not support the "outrageous discrimination in our constitution against Roman Catholics and equally unfair treatment of women".
He argued that it was "not acceptable in this day and age" that the Princess Royal was lower down the line of succession than her younger brothers Andrew and Edward. He described the bar on Catholics marrying into the Royal Family as "offensive" and an "antediluvian measure".
A survey by ICM for the BBC yesterday showed 89 per cent of voters backed giving female heirs equal succession rights and 81 per cent believed heirs who married Catholics should still be able to accede to the throne.
So, what if Victoria had been followed by her daughter ...
*Were it not for the role of male primogeniture in the rules governing the succession, the British monarchy and the course of the First World War might have been very different, thanks to the fact that Queen Victoria's first child was a girl.
The child, also called Victoria, was prevented from succeeding to the throne by the clause in the 1701 Act of Settlement which dictates that the first-born son of the monarch takes precedence over any daughter.
The result was that Queen Victoria's feckless playboy son, Edward, supplanted his sister to become king as Edward VII and, arguably, ended an alliance that could have avoided the First World War.
At the time of her mother's death, Victoria was already married to Prince Frederick William of Prussia, who became the German Kaiser Frederick III. If Victoria, who retained her title as Britain's Princess Royal, had become Queen, her son, Kaiser Wilhelm II of Germany, would have become King Wilhelm I of Great Britain, in effect uniting the royal families of the two countries.
It is therefore possible that with Wilhelm remaining as Kaiser in 1914, the carnage of the French battlefields could have been avoided. Such revisionism, of course, pays no heed to the problems of a political alliance between the competing imperial ambitions of Britain and Germany. Indeed, when hostilities broke out in 1914, King George V was forced to contest whispers that he was not entirely in favour of the war against the forces of his first cousin, Wilhelm II.
When the author HG Wells spoke of the royal court as "alien and uninspiring", the British monarch responded: "I may be uninspiring, but I'll be damned if I'm an alien."
...or the Stuarts still ruled
*The key purpose of the Act of Settlement was to head off any claims from the deposed Roman Catholic king, James II, and any of his co-religionist heirs. If the law had not been passed, the heir to the British throne would be a 75-year-old Bavarian prince who survived the Nazi death camps.
Franz, the current Duke of Bavaria, right, is considered the current head of the House of Stuart, started by Charles I and continued – after the English Civil War, his father's decapitation, and the Interregnum – by his second son, James II. When James, the last Catholic ruler of England, Wales and Scotland, abdicated his throne, the Stuart line passed through the royal families of Savoy and Austria before settling in the House of Wittelsbach, the ruling family of the kingdom of Bavaria.
Franz came from a family opposed to the Nazi regime. In 1944, his family was arrested and sent to a series of concentration camps, including Dachau. He currently lives in the Nymphenburg Palace in Munich.On his death, the title will pass to his niece, Sophie, Princess of Liechtenstein. The House of Windsor can, however, rest easy. Franz does not press his claim on Buckingham Palace and has always been on good terms with the Royal Family.