Perkin Warbeck (circa 1474 – 23 November 1499) was a pretender to the English throne during the reign of King Henry VII of England. By claiming to be Richard of Shrewsbury, 1st Duke of York, the younger son of King Edward IV, one of the "Princes in the Tower", Warbeck was a significant threat to the newly established Tudor dynasty, and gained support outside England.
Henry VII declared Warbeck an impostor and after his capture Warbeck wrote a confession in which he said he was a Fleming born in Tournai around 1474.
Due to uncertainty as to whether Richard of Shrewsbury had died in the Tower of London or had survived, Warbeck's claim gathered some followers, whether due to real belief in his identity or because of desire to overthrow Henry and reclaim the throne. Dealing with Warbeck cost Henry VII over £13,000 putting a strain on Henry's weak state finances.
More information on the Wikipedia page . There is much other coverage on the web.
See also Lambert Simnel