The Act 11 Hen 7 c 1 (sometimes informally referred to as the Treason Act 1495 or as the Rex de facto statute) is an Act of the Parliament of England which was passed in the reign of Henry VII of England. The long title of the Act is "An Acte that noe person going wth the Kinge to the Warres shalbe attaynt of treason." The Act states that a person serving the king de facto for the time being is not guilty of treason, or of any other offence, if he wages war against the king de jure. William Blackstone wrote that the Act is "declaratory of the common law." It is still in force.
Henry had become king after defeating Richard III in the Battle of Bosworth on August 22, 1485. However he backdated the start of his reign to August 21, the day before the battle, enabling him to prosecute anyone who had fought under his rival, and to execute them for treason. This was highly controversial at the time, since it meant that, in a future battle, anyone who fought for the rightful king against a usurper would be at risk of execution if they lost, and this might undermine their courage in battle and their loyalty to their king. Nevertheless, Henry VII had his way at the time as Parliament was then in no position to oppose him (although later that year a general pardon was issued to those who had fought for Richard).
However, ten years later Henry's position on the throne was sufficiently secure that he could afford to grant Parliament what they wanted, when in 1495 they passed a bill to prevent the treason laws from being abused in this way again. The resulting Act is still in force today, and was applied to Scotland in 1708.
The Act was cited by Sir Harry Vane in his treason trial in 1662 following the Restoration. He was one of those accused of serving with Oliver Cromwell against the king during the English Civil War, and in his defence he relied upon the Act. However the court ruled that the 1495 Act was only intended to protect those who fought for a king, not to protect republican rebels who fought to abolish the monarchy. He was convicted and executed.
In New Zealand, section 64 of the Crimes Act 1961 provides that obedience to the laws of a person with "possession de facto of the sovereign power" is protected from criminal responsibility.
Leonard Casley of the Principality of Hutt River (an unrecognised micronation in Australia) used this Treason Act to his advantage by declaring himself a prince, thereby purporting to protect him from prosecution by the Australian government.