If Tony Blair has been reticent about converting to Catholicism while Prime Minister, it may be because of Northern Ireland. After all, there has not been a restriction on a prime minister’s faith since the reign of George IV.

With the restoration of the monarchy in 1660 came measures reenforcing the supremacy of the Church of England. Only those who received the Anglican rites of Holy Communion could hold office. By 1678 MPs and peers had to state that they renounced transubstantiation and the invocations of the Catholic Mass as “superstitious and idolatrous”.

In the Act of Settlement 1701, Catholics, and those married to Catholics, were barred from the throne “for ever”.

The Catholic Emancipation Act 1829 permitted Catholics to enter Parliament and hold any office except that of monarch, Lord Chancellor, Regent, Lord Lieutenant of Ireland and judicial roles in ecclesiastical courts.

But the floodgates hardly opened. There were very few practising Catholics in Regency Britain, apart from the Irish. Furthermore, fear and prejudices often cut both ways. The Catholic clergy initially continued to denounce Catholics attending Oxford and Cambridge after the two universities scrapped their religious entry requirements.

Harold Macmillan came close to converting after leaving Eton, but decided at the last moment not to follow Ronnie Knox, his influential former tutor. Knox became a monsignor. In old age, Macmillan told his biographer, Alistair Horne: “If things had been otherwise, I suppose it not impossible that Ronnie might have become Prime Minister and I should have been Mgr Macmillan!” Perhaps a similar thought struck Mr Blair.

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