Sources

1) Source: The Berkeley Estate "Kingswood Abbey and Sir Maurice lord of Berkeley, son of Thomas de Berkeley"Dated: 1243-1246. External Link
http://discovery.nationalarchives.gov.uk/details/r/cd2de2c4-037c-4cbe-84a3-de111babeaa2. Reference: BCM/A/1/48/6 (Data from direct source)
2) Source: English Baronies: a Study of Their Origin and Descent, 1086-1327. Reference: Pages 13-14: Berkeley (Questionable reliability of evidence)
3) Source: The Berkeley Estate "Thomas has granted to the priory the land which John Blunt held of him; and has also confirmed a rent of 24s. in the vill of B. . . [which was granted to them by?] Maurice de Berkeley his father...Witnesses: . . . son of Richard, Peter de Stintescumbe, Richard de Couele, Thomas de ..., Robert de Couele, Robert de la Planke, Hugh the baker (pistor),...". External Link
http://discovery.nationalarchives.gov.uk/details/rd/76d1a4d9-8c6f-4fe9-b690-4d5a91d7af58. Reference: BCM/A/1/67/2 (Data from direct source)
4) Source: Chancery: Inquisitions Post Mortem, Series I, Edward I "Maurice de Berkeley...Thomas his son, aged 30, is his next heir...Slinbrugg. The manor, held of Henry de Berkele, service unknown because it was given in free marriage to a predecessor of the said Maurice."Dated: 5 April 1281. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol2/pp230-238. Reference: C 133/28/11 (Data from direct source)
5) Source: The Berkeley Estate "The prior has granted to Thomas an anniversary in full choir [etc.] for the souls of Maurice, Thomas's father, and Isabel his mother and, after their deaths, for the souls of himself, Joan his wife and Maurice their son"Date: 24 February 1308. External Link
http://discovery.nationalarchives.gov.uk/details/r/c8a0e990-6cf7-4ba2-9921-e4027898c8d6. Reference: BCM/A/1/19/29 (Data from direct source)
6) Source: The metrical chronicle of Robert of Gloucester Sir Richard fiz le rei - of wan we speke biuore - Gental man was inou - thei he were abast ibore - vor the erles doughter of wareine - is gode moder was - & is fader the king Ion - ... - Sire morrisse of berkeleye - weddede suththe bicas - is doghter and bighet on hire - the knight sir tomas - ... - Tuelf hundred in the gher of grace - it was & eightene. Reference: 10,626 (Data from secondary evidence)