Lord James Douglas Hamilton, Tory whip in the Scottish Parliament, wrote to Mr Blair asking him to lift of the 1700 Act of Settlement, which he described as an "anachronistic and anomalous injustice".

The Act, passed by the English Parliament, bars royals from becoming King or Queen if they "profess the popish religion or shall marry a Papist".

But Mr Blair told Lord James the Government had "no plans" to repeal the law because attempts to amend it would be "complex in the extreme", requiring amendments of other legislation in Britain and in several Commonwealth countries.

The PM, whose wife Cherie is a Roman Catholic, added: "The central point of the Act of Settlement is that the Established Church in England is the Church of England, of which the Sovereign is Supreme Governor.

"Therefore the Act does not prevent members of the Royal Family from becoming or marrying Roman Catholics, but does remove them from the line of succession."

'Multi-faith nation'

But Lord James plans to seek the views of Scottish churches with the aim of gathering support for a repeal of the law.

The MSP for the Lothians said: "It would seem that a combination of fear, ultra-caution, legislative complexity and inertia would have Tony Blair's regime adopt a do-nothing policy - with the result that this unfair and unjust anomaly is to continue.

"Britain is now a multi-faith nation and yet British law prescribes that the heir to the throne cannot succeed to the throne and be married to a Catholic.