The Queen’s grandson Peter Phillips could be forced to renounce his place in the line to the British throne if he goes ahead and marries his Canadian fiancee.

Mr Phillips, who is the son of the Princess Royal and her first husband Captain Mark Philips, is tenth in line to the throne but his marriage could cost him his birthright because his fiance, Autumn Kelly, is a Roman Catholic.

The Tablet , the Catholic weekly, reveals in this week’s edition that Miss Kelly, the daughter of Brian and Kathleen “Kitty” Kelly, was baptised on 18 June 1978 at St John Fisher Parish church in Pointe-Claire, a suburb of Montreal in Quebec.

Although it is not clear whether she is still a practising Catholic, unless she agrees to renounce her religion the union will fall foul of the 1701 Act of Settlement, which bars the monarch or heirs to the throne from marrying a Catholic.

The Act discriminates uniquely against Catholics. Although a person must be in communion with the Church of England to succeed to the throne, there is nothing in the Act which would prevent a monarch or their heirs from marrying a Hindu, Buddhist, Muslim or Jewish person.

Mr Phillips would be the second Royal placed in this dilemma by the Act, which has for centuries been a cause of anguish to Catholics.

Prince Michael of Kent relinquished his place in the succession when he married a Catholic. He and Princess Michael raised their two children as Anglicans to ensure the same fate did not befall them.

There is growing anger in Catholic circles that Britain’s increasingly trenchant anti-discrimination laws do not extend to their denomination.

Cardinal Keith O’Brien, Catholic Archbishop of Edinburgh, told The Tablet the Act was “scandalous” and said it was wrong for Mr Phillips to have to do this.

The engagement was announced last week but no date has yet been set for the marriage.

Cardinal O’Brien said he had every sympathy for Mr Phillips, who is as low-profile as it is possible for a member of Britain’s Royal family to be, and who works for the Royal Bank of Scotland. He also sympathised with Miss Kelly.

“Whether a person be fortieth or second in line to the throne, it is wrong that they be deprived of that right because they have fallen in love and chosen to marry a Roman Catholic,” he told The Tablet.

“It doesn’t matter if the Catholic is not practising the faith or the person in line doesn’t want the throne, it is wrong that he or she is deprived of their birthright by this scandalous Act which should not be on our statute book.”

Cardinal O’Brien had earlier criticised Prime Minister Gordon Brown for failing to address the discrimination against Catholics in the Act of Settlement when he announced his plans for constitutional reform in the first days of his premiership.

According to the journal, Catholic MP John Gummer, whose Ten Minute Rule Bill earlier this year sought to overturn the remaining anti-Catholic legislation, had understood that Mr Brown had changed his plans at the last minute, and was foolish to do so.

Mr Gummer said: “I know for a fact that up until a day or two before Mr Brown delivered his statement he had been willing to put my bill into law. The effect of this ridiculous law is now going to be felt. The best thing would have been to change the law when it did not apply to anybody rather than changing it when it applies to an individual. It is unacceptable that the part of the Christian church that has more active adherents than any other should be discriminated against in this way.”

Mr Gummer accused “right-wing nominal Anglicans” who were virulently anti-Catholic of persuading Mr Brown that it would particularly bad for him as a Scot to undermine the Protestant Church of England, of which the Queen is Supreme Governor.

The Archbishop of Westminster, Cardinal Cormac Murphy-O’Connor, who last month said he would urge Mr Brown to repeal the Act of Settlement when he meets him later this year, declining to comment on Miss Kelly’s religion. A spokesman said he wished the couple “every happiness”.

Tablet editor Catherine Pepinster said the Act of Settlement, passed to disqualify the Stuart Pretenders, would not prohibit a Catholic from exercising the Royal Prerogative as Prime Minister. The law is a restriction on the private lives – and consciences – only of members of the royal family. “But it is also the last symbol of Britain’s anti-Catholic history, as the Act’s archaic and offensive reference to ‘papists’ makes clear,” she writes in a leader column.

Read The Tablet article