With the upcoming marriage of Peter Phillips to his Canadian older woman, Autumn Kelly - an Irish Catholic from Montreal - the Act of Succession once again comes into force.

In recent months we've seen more public debate about what is essentially a ban on members of the Royal Family marrying a Catholic partner whilst retaining their place in the line of succession.

British Prime Minister Gordon Brown has not deemed this a priority issue so let us indulge in a purely academic parlour game.

What if Catholic Succession were allowed?

For starters, the dressmakers would need to make a new dress for the Vatican City, for Catholic Queens have the right to wear white (Privilege du blanc) during audiences with the Holy Father, whereas all other ladies should wear black. (Not for the first time in her life, Tony Blair's wife, practicing Catholic Cherie, broke protocol by wearing white during her meeting with Pope Benedict XVI in 2006).

More seriously, though, there would be a need to update the line of succession to include, for instance, the names of the Kent boys, the sons and grandchildren of the present Duke and Duchess of Kent (Edward and Katherine).

At present, the couple's only Protestant descendants are borne of the union of their daughter, Lady Helen Taylor (left) and her art gallery husband, Tim Taylor.

And what about Prince Michael of Kent, whose children Lord Frederick and Lady Gabriella were born Protestant?

Before this summer's engagement of the Queen's grandson, Peter Phillips, about whom it's yet to be announced that he'll be forced to renounce his position in the line of succession, Prince Michael was perhaps the most high profile member of the Royal Family to do exactly this upon his marriage to the elegant Marie-Christine in 1978.