In 1785, they secretly contracted a marriage that was invalid under English civil law because his father, King George III, had not consented to it.
She was a Roman Catholic and so had the marriage been approved and valid, George would have lost his place in the line of succession since the law then forbade Catholics and spouses of Catholics from becoming monarch.
Before marrying the prince, Fitzherbert had been twice widowed.
Her nephew-in-law from her first marriage, Cardinal Weld, persuaded Pope Pius VII to declare the marriage sacramentally valid.