Gordon Brown is considering repealing the 1701 Act of Settlement as a way of healing a historic injustice by ending the prohibition against Catholics taking the throne.
But doing so would have the unforeseen consequence of making a 74-year-old German aristocrat the new King of England and Scotland.
Without the Act, Franz Herzog von Bayern, the current Duke of Bavaria, would be the rightful heir to the British Crown under the Stuart line.
The bachelor, who lives alone in the vast Nymphenberg Palace in Munich, is the blood descendant of the 17th-century King Charles I.
"If it [the Act] goes then the whole Catholic line is reinstated," said Prof Daniel Szechi, a lecturer in early modern history at the University of Manchester.
"Franz becomes the rightful claimant to the throne. We would just exchange one German family for another one."
The Act was introduced as part of the power struggle between Parliament, the Christian churches and the monarchy, then dominated by the House of Stuart.
It prohibits any Roman Catholic from having access to the throne, even through marriage. Once a person marries a "Papist" they shall be "for ever incapable to inherit, possess or enjoy the Crown", it asserts.
The legislation effectively severed the Stuart line of succession, a family who favoured Catholicism, and switched it to their distant relatives the Hanoverians, from which our current Queen descends. James II, the son of King Charles, fled into exile. - tg