The Act of Settlement, which prevents the monarch marrying a Catholic, was drawn up in a time of religious anarchy in the 18th century.Now 308 years later, Lib Dem MP Dr Evan Harris is trying to have it reformed.
Since the reign of King Henry VIII in the 16th century, the ruler of England has also been supreme governor the established church - Anglicanism.
But in 1701, King William III felt the law needed to go further to restrict the monarch's religion.
He was ill and childless and was desperate to ensure his rival James II, an exiled Catholic, would not return to the throne.
So he drew up the Act of Settlement, which states that anyone who "shall profess the popish religion, or shall marry a papist" cannot be monarch.
Crucially, the act does not ban the monarch from marrying anyone of any other religion or indeed someone of no faith at all.
Dr Harris believes that the continued existence of this law is a form of unacceptable discrimination "written into the UK's constitution".
"It seems to be patently unjustified to place a restriction on Catholics but not Jews, Muslims or atheists," he told the BBC News website.
Princess Michael
The argument goes that a change in the law would weaken the position of the Church of England - a Catholic monarch could hardly swear to defend a faith that was not his own.
Dr Harris disputes this because he does not want to change the requirement that the sovereign themselves must be Anglican. The issue for him is who they may marry.
Others opposed to reform - including Tony Blair - have pointed out that the act does not prevent members of the Royal Family marrying whomever they choose. It simply removes them from the line of succession if that person is a Catholic - and that only an issue for those closest to the top job.
But the matter has been an issue for several members of the Royal Family in recent years.
In 1978, Prince and Princess Michael of Kent were married. She was a Catholic and steadfastly refused to convert to Anglicanism.
Consequently, Prince Michael, who was 15th in line to the throne, lost his place in the succession.
Their children, however - Lord Frederick and Lady Gabriella Windsor - retain their rights of succession because they are still "in communion with the Anglican Church".
In another case, in 1994, the Duchess of Kent converted to Catholicism, but her husband, the duke, did not lose his place in the succession because she was an Anglican at the time of their marriage.
Most recently, when Peter Philips, the Queen's oldest grandchild, became engaged to his Catholic fiancée Autumn Kelly, she converted to Anglicanism so that he did not have to forfeit his place of 11th in line to the throne.
Damian Thompson, editor-in-chief of the Catholic Herald newspaper, said: "What the average Catholic finds offensive is the idea that if Prince William wants to marry a Catholic, he can't, but he can marry a Scientologist or a Holocaust denier.
"I suppose it's understandable for historical reasons, but that's really offensive."
He said, however, that while most Catholics, if asked, would want the law to be changed, it was not really "a burning issue".
But he added: "Where I think people do feel more strongly about it is in Scotland where Catholics and Protestant don't get on. It is seen there as a relic of Britain's sectarian past."
'An anomaly'
In 1999, former Scottish secretary for the Conservatives Lord Forsyth failed in his bid to overturn the act, which he called Britain's "grubby little secret".
Former Archbishop of Glasgow Cardinal Thomas Winning has also called the law an "embarrassing anachronism" and demanded it be amended.
The Catholic Church in England and Wales takes a less strident view.
A spokesman for its head, Cardinal Cormac Murphy-O'Connor, said: "His position is that it is an anomaly he is sure will be repealed in the future.
"It's not something that is top of our agenda. There's a lot of other legislation going through that would have far more impact on how the Catholic church operates in this country, such as the assisted dying bill and the human fertilisation and embryology bill."
While it is "strange" that it applies only to Catholics not any other faith, that is more indicative of Britain's history than any kind of discrimination, the spokesman added.
But not everyone believes the prospect of a change in the law is innocuous.
In 2005, then Conservative leader Michael Howard sacked his parliamentary candidate Adrian Hilton for comments he made in an article about the Act of Settlement two years earlier.
Mr Hilton was accused of being a "bigot" after claiming the act was passed because "the nation had learnt that when a Roman Catholic monarch is upon the throne, religious and civil liberty is lost".
He argued that without "the protective barrier of Anglican establishment" Britain would lose its "foundation as a Christian nation".
Complex process
The UK is not alone in placing restrictions on the religion of its monarchy. In the Netherlands, for example, all monarchs must be Protestant and in Spain, succession is strictly through Roman Catholic houses.
Changing the status quo would also not be straightforward.
Amending the act would require changes to as many as nine other laws and would also require a great deal of co-ordination with Commonwealth countries, who would need to change their own legislation.
Last year, Justice Secretary Jack Straw said the government was "ready to consider" changing the law.
But today a Ministry of Justice spokesperson said: "The government has always stood firmly against discrimination in all its forms, including against Roman Catholics, and we will continue to do so.
"We are examining this complex area although there are no immediate plans to legislate."
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(Source: BBC)