THE former London Welsh and Swansea No 8 won 38 caps and toured with the British and Irish Lions to New Zealand in 1971 and South Africa three years later, featuring in eight Tests.
Known throughout the rugby world as 'Merv the Swerve', he suffered a brain haemorrhage playing for Swansea against Pontypool in a Welsh Cup semi-final in 1976 that cut short his career.
A statement released by the WRU this morning said: "The Welsh Rugby Union is saddened to learn of the death of Mervyn Davies who deservedly earned a worldwide reputation as one of the greats of the modern game.
"Flags at the Millennium Stadium are flying at half-mast from today in honour."
The WRU said Davies passed away following a long illness.
The current Wales team will be crowned Six Nations champions and Grand Slam winners if they beat France at the Millennium Stadium on Saturday.
Leading the tributes, WRU president Dennis Gethin said: "We have lost a great player, a wonderful ambassador for the game and a true gentlemen.
"I played against Mervyn many times and knew just how good he was, but I also grew to appreciate him as a true friend.
"In later life he also became an accomplished after dinner speaker, so his loss will be felt in many ways by so many people."
WRU chairman and former Wales skipper David Pickering added: "Mervyn Davies was a man who epitomised the values of Welsh rugby and will be sadly missed by everyone who knew him and all who knew of him.
"He was a giant of the game, both on and off the field, for London Welsh, Swansea, Wales, the Barbarians and the British and Irish Lions.
"He bore his illness with courage, and his memory will undoubtedly live on within our game for all time."
And WRU group chief executive Roger Lewis said: "We have lost a true great of our game with the sad death of Mervyn Davies.
"His loss will be felt across the rugby world because of the huge impact he had as an icon of the game.
"He stood out in one of our great Welsh teams. but remained a modest and gentle man off the field of play throughout his life.
"Mervyn also played a prominent role in Welsh international rugby's former players' association. so his love of the game remained undiminished throughout his life.
"It is right and fitting that this weekend his memory will be honoured by everyone involved in Welsh rugby."
Davies made his Wales debut against Scotland at Murrayfield in 1969, and led Wales to a Five Nations Grand Slam seven years later.
He played for London Welsh from 1968, and later joined Swansea. where he completed his playing career.