Anti-Catholicism in the United Kingdom dates back to the martyrdom of Saint Alban in the Roman era. Attacks on the Church from a Protestant angle mostly began with the English and Irish Reformations which were launched by King Henry VIII and the Scottish Reformation which was led by John Knox. Within England, the Act of Supremacy 1534 declared the English crown to be "the only supreme head on earth of the Church in England" in place of the Pope. Any act of allegiance to the latter was considered treasonous because the papacy claimed both spiritual and political power over its followers. Ireland was brought under direct English control starting in 1536 during the Tudor conquest of Ireland. The Scottish Reformation in 1560 abolished Catholic ecclesiastical structures and rendered Catholic practice illegal in Scotland. Today, anti-Catholicism remains present in the United Kingdom, particularly in Scotland and Northern Ireland.
Anti-Catholicism among many of the English was grounded in the fact that the Holy See sought not only to regain its traditional religious and spiritual authority over the English Church, but was also covertly backing regime change in alliance with Philip II of Spain as a means to ending the religious persecution of Catholics throughout the British Isles. In 1570, Pope Pius V declared Elizabeth I who ruled England and Ireland deposed and excommunicated with the papal bull Regnans in Excelsis, which also released all Elizabeth's subjects from their allegiance to her. This rendered conditions impossible even for Elizabeth's subjects, like Richard Gwyn and Robert Southwell, who were completely apolitical but persisted in their allegiance to the Catholic Church in England and Wales, as the Queen and her officials refused to accept that her subjects could maintain both allegiances at once. The Recusancy Acts, legally coercing English, Welsh, and Irish citizens to conform to Anglicanism and attend weekly services on pain of prosecution for high treason, date from Elizabeth's reign. Later, regicide and decapitation strike plots organized by persecuted Catholics were heavily exploited by the Crown for propaganda and further fuelled anti-Catholicism in England. In 1603, James VI of Scotland became also James I of England and Ireland.
