Revered in his homeland, he was seen as a "New Zealand hater" Down Under because of his often vehement denunciation of their style of play. He was highly critical of the way the 1959 Lions lost the Test series in New Zealand, when the All Blacks won the first of four matches 18-17 thanks to six penalty goals kicked by Don Clarke to four tries by the Lions. He accused the New Zealanders of cheating when they beat Wales 13-12 at Cardiff Arms Park in the second Test of their Grand Slam tour of the British Isles in 1978.
Thomas's astute eye caught the full sequence of the diving out of the line-out by the All Blacks' locks, Frank Oliver and Andy Haden, which earned their side a match-winning penalty. Having seen Wales defeat New Zealand in 1935, and reported on another outstanding Welsh victory in Cardiff in 1953, Thomas, like so many other Welsh rugby lovers, had thought the long wait for another success was about to be ended.
Then came the dive, a Brian McKechnie penalty and a bitter defeat for Wales. Thomas's front page accusations about the cheating All Blacks were flashed around the world. Significantly, they were later borne out in Haden's autobiography, which chronicled the plan to dupe the English referee, Roger Quittenton. The incident highlighted Thomas's ability to tackle hard-news stories as well as provide informed comment on players and matches. Throughout them all he was scrupulously fair.
Born in Pontypridd, Thomas's lifelong passion for rugby was launched by his father, when he took him to see Wales play in the 1920s. Although Thomas's first job was as a civil servant at the Cardiff City Hall he was an avid writer even then, regularly sending match reports and features to local newspapers.
He joined the Royal Navy during the Second World War and, after being commissioned in the RNVR, rose to the rank of First Lieutenant on a minesweeper. He retained an interest in naval history and ships throughout his life, although his real passion was trains.
He joined the Western Mail, the national newspaper of Wales, in 1946 as chief rugby writer and quickly made an impact. He was appointed sports editor in 1965 and became assistant editor of the paper in the late Seventies.
He became the paper's first writer to cover a Lions tour, when he spent more than three months away with the 1955 party in South Africa. That was the first of eight successive Lions tours that he covered for the paper and he followed up each one with a book. In all he wrote 28 books on rugby between 1954 and 1980 and was appointed MBE for his services to journalism in 1984.
Stories of his generosity to players on tour were legion, anecdotes about his ability to entertain legend. Thomas - known as "Bryn" (short for Brinley) - was the doyen of rugby writers, not merely in Wales, and was a former chairman, as well as a founding member, of both the Rugby Union Writers' Club and the Welsh Rugby Writers' Association.
The effect his writing was able to generate is best illustrated by the fact that his editor once told him to "make a few mistakes" when predicting the Welsh team so that the readers of the Western Mail would not think it was he who picked the national side.
"Like all young players who wanted to succeed in the game my first thought on a Monday morning was to open the paper and see what J.B.G. had said", said the former Wales and Lions scrum-half Gareth Edwards. "His affinity with players was strongly forged and many of us looked upon him as a father figure." No greater tribute could be offered to any sports writer.
John Brinley George Thomas, journalist: born Pontypridd, South Glamorgan 29 April 1917; married 1941 Gwen Owen (died 1985; three sons); died 11 April 1997.