His former Scotland team-mate Scott Hastings, whose mother-in-law died from the illness in recent years, said he was “in tears” when Weir phoned to tell him about his condition a few weeks ago.
“It is an awful, cruel disease,” Hastings told BBC Radio Scotland. “But Doddie, like the rest of his life, has approached everything with a real relish to challenge the disease.
“He’s one of the great characters of Scottish rugby.”
Craig Stockton, chief executive of MND Scotland, spoke of his sadness at learning of Weir’s condition.
“This is devastating news for anyone but especially for someone with a young family and for whom physical fitness has been such an important part of his life,” he said.
“It is very brave of him to come forward publicly to share his diagnosis and raise awareness of the illness. MND Scotland will be here to support him and his family and all those affected by MND in Scotland.”