The future King George III is alleged to have married secretly, on 17th April 1759, a Quaker, Hannah Lightfoot, the daughter of a Wapping shoemaker, who is said to have borne him three children.
Documents relating to this marriage, which bore the Prince's signature, were impounded and examined in 1866 by the Attorney General. Learned opinion at the time leaned to the view that the documents were genuine.
They were placed in the Royal Archives at Windsor. Permission to view them by a would-be author was refused in 1910, and no outsider has seen them since.
If the future King George III did make such a marriage it would have been legal as it was before the Royal Marriages Act (1772.) Therefore, his marriage to Princess Charlotte in 1761 was bigamous.
Page 286. 'Britain's Royal Families: The Complete Genealogy'. Alison Weir. 2002.