The Prayer Book Rebellion, Prayer Book Revolt, Prayer Book Rising, Western Rising or Western Rebellion (Cornish: Rebellyans an Lyver Pejadow Kebmyn) was a popular revolt in Devon and Cornwall in 1549. In that year, the Book of Common Prayer, presenting the theology of the English Reformation, was introduced. The change was widely unpopular – particularly in areas of still firmly Catholic religious loyalty (even after the Act of Supremacy in 1534) such as Lancashire. Along with poor economic conditions, the attack on the Catholic Church led to an explosion of anger in Devon and Cornwall, initiating an uprising. In response, Edward Seymour, 1st Duke of Somerset sent Lord John Russell with an army composed partly of German and Italian mercenaries to suppress the revolt.

One probable cause of the Prayer Book Rebellion is the religious changes recently implemented by the government of the new king, Edward VI. In the late 1540s, Lord Protector Somerset, on behalf of the young king, introduced a range of legislative measures as an extension of the Reformation in England and Wales, the primary aim being to change theology and practices, particularly in areas of traditionally Roman Catholic religious loyalty - for example, in Cornwall and Devon.

When traditional religious processions and pilgrimages were banned, commissioners were sent out to remove all symbols of Catholicism, in line with Thomas Cranmer's religious policies favouring Protestantism ever more. In Cornwall, this task was given to William Body, whose perceived desecration of religious shrines led to his murder on 5 April 1548, by William Kylter and Pascoe Trevian at Helston.

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