Gordon Brown today insisted he was determined to remove "discrimination" after it emerged the Government had opened talks with Buckingham Palace over laws preventing heirs to the throne from marrying Roman Catholics.
The Prime Minister stressed there were no "easy answers" to changing the 1701 Act of Settlement.
But, in a round of broadcast interviews during his pre-G20 travels, he indicated he would be raising the issue with other Commonwealth leaders when th
ey gather in November.
"This is a very complex issue that has been a matter of controversy and discussion for decades, indeed over centuries," Mr Brown said.
"What we must do is protect the position of the monarchy and the position of the Queen as head of the established church, the Church of England.
"But there are clear issues about the exclusion of people from the rights of succession, and there are clearly issues that have got to be dealt with not just in Britain but right across the Commonwealth.
"This is not an easy set of answers, but I think in the 21st century people do expect discrimination to be removed and they do expect us to be looking at these issues."
Ministers have already signalled a willingness to address the issue, which will be raised in the Commons today when MPs debate legislation introduced by a Liberal Democrat MP to end the "uniquely discriminatory" rules.
Evan Harris has cross-party support for his proposals but looks unlikely to win ministerial support at this stage as the Government grapples with what it said would be a "complex undertaking".
The Act states that heirs to the throne lose their right to be the sovereign if they marry a Catholic or convert – forcing royal brides over the years to leave the faith to protect their husband's birthright.
Autumn Kelly, the wife of Peter Phillips, the Queen's grandson, did just this before their wedding last year so that he remained 11th in line to the throne.
His claim could be made stronger still if action to end the practice of male heirs taking precedence was made retrospective as his mother, the Princess Royal, would leapfrog her two younger brothers.
She would become fourth in line, behind Prince Harry, instead of coming after the Duke of York and the Earl of Wessex and their children.
Dr Harris's Royal Marriages and Succession to the Crown (Prevention of Discrimination) Bill is co-sponsored by Catholic parliamentarians such as Tory MP Edward Leigh, Labour's Andrew Mackinlay and John Grogan and fellow Liberal Democrat John Pugh.
He 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."
Asked if it would back the backbench legislation, 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."
A Downing Street spokeswoman said: "This is a complex issue. While there is no question of changing the constitutional role of the Monarch or of changing the role of the Church of England as the established church, people expect the Government to look at issues of discrimination.
"The laws concerning marriage to Catholics and the primacy of male members of the Royal family should change, but that can only happen with the agreement of the Palace and of all the Commonwealth countries of which HM The Queen is the head of state.
"We are keen to open a process of dialogue with them which can lead to changes in this legislation, but that cannot happen overnight, which is why we cannot support this Private Members Bill."
Dr Harris welcomed the findings of an opinion poll which found overwhelming public support for removing the discrimination against Catholics and female royals.
The survey, by ICM for the BBC, showed 89% of voters backed giving female heirs equal succession rights and 81% believed heirs who married Catholics should still be able to take to the throne.
Despite continued support for the monarchy by more than three quarters of the public for the monarchy, almost one in five (18%) said they would prefer to see Britain become a republic when the present Queen dies.
The Lib Dem MP said: "Very few political ideas, let alone constitutional changes, have two-thirds support yet ending discrimination against Catholics in royal marriages and against women in the succession both have over 80% support.
"Even more people believe that the monarchy should be fair than believe that there should be a monarchy at all so the Government and the Conservative Party should support this bill and end these historic injustices as soon as possible."
ICM Research interviewed a random sample of 1,005 adults across the country by telephone between March 20-22 and the results were weighted to reflect the population.
