By the time Margaret, her son and daughter-in-law (Anne) were ready to follow Warwick back to England, the tables had again turned in favour of the Yorkists, and the Earl was defeated and killed by the returning King Edward IV in the Battle of Barnet on 14 April 1471. Margaret was forced to lead her own army at the Battle of Tewkesbury on 4 May 1471, at which the Lancastrian forces were defeated and her seventeen-year-old son Edward of Westminster was killed. The circumstances of Edward's death have never been made clear; it is not known whether he was killed in the actual fighting or executed after the battle by the Duke of Clarence. Over the previous ten years, Margaret had gained a reputation for aggression and ruthlessness, but following her defeat at Tewkesbury and the death of her only son, she was completely broken in spirit. After she was taken captive by William Stanley at the end of the battle, Margaret was imprisoned by the order of King Edward. She was sent first to Wallingford Castle and then was transferred to the more secure Tower of London. Henry VI was also imprisoned in the Tower in the wake of Tewkesbury and he died there on the night of 21 May; the cause of his death is unknown, though regicide was suspected. In 1472 she was placed in the custody of her former lady-in-waiting Alice Chaucer, Duchess of Suffolk, where she remained until ransomed by Louis XI in 1475.