William Ros or Roos, 1st Baron Ros of Helmsley (c. 1255 – 6 or 8 August 1316), was one of the claimants of the crown of Scotland in 1292 during the reign of Edward I. Family William Ros was the eldest son of Robert de Ros (d. 17 May 1285) of Helmsley, Yorkshire, and Isabel d'Aubigny (c. 1233 – 15 June 1301), daughter and heiress of William D'Aubigny of Belvoir, Leicestershire, and granddaughter of William d'Aubigny. He had four brothers and three sisters: Sir Robert Ros of Gedney, Lincolnshire. John Ros. Nicholas Ros, a cleric. Peter Ros, a cleric. Isabel Ros, who married Walter Fauconberg, 2nd Baron Fauconberg. Joan Ros, who married John Lovell, 1st Baron Lovell. Mary Ros, who married William de Braose, 1st Baron Braose. Career On 24 December 1264, William's father, Robert de Ros (d. 1285), was summoned to Simon de Montfort's Parliament in London as Robert de Ros, and for some time it was considered that the barony was created by writ in that year, and that Robert de Ros was the 1st Baron Ros. According to The Complete Peerage: In 1616, the barony of Ros was allowed precedence from this writ [of 24 December 1264], a decision adopted by the Lords in 1806 (Round, Peerage and Pedigree, vol. i, pp. 249-50); but these writs, issued by Simon in the King's name, are no longer regarded as valid for the creation of peerages. Accordingly, the barony is now considered to have been created when William Ros was summoned to Parliament from 6 February 1299 to 16 October 1315 by writs directed Willelmo de Ros de Hamelak. William Ros succeeded to the family honours and estates on the death of his mother. He was an unsuccessful competitor for the crown of Scotland, founding his claim on his descent from his great-grandmother, Isabel, a bastard daughter of William I of Scotland. He was buried at Kirkham Priory. He was involved in the wars of Gascony and Scotland. He discovered that Robert de Ros, Lord of Werke, intended to give up his castle to the Scots. William notified the king of this, who sent him with a thousand men to defend that place. The place was then forfeited because of the treason of Robert de Ros. William Ros then took possession of it. William was appointed warden of the west Marches of Scotland. Through his marriage to Maud de