Cathy Ferguson, the wife of former Manchester United manager Sir Alex Ferguson, died at the age of 84.
The two married in 1966, where they spent 57 years as husband and wife, and had three sons, including Peterborough manager Darren Ferguson.
Flags have been lowered at Old Trafford as a mark of respect, and both the men's and women's teams will wear black armbands for their matches this weekend.
Manchester United said in a statement: "Everyone at Manchester United sends our condolences to Sir Alex Ferguson and his family on the passing of Lady Cathy, a mother, sister and grandmother, who was a pillar of strength. to Sir Alex throughout his life."
Cathy and Sir Alex, 81, met in 1964 while they were both working in a bicycle factory.
Sir Alex was Manchester United manager for 27 years together and Cathy is said to have played a key role in convincing him not to retire in 2002.
When Sir Alex announced his retirement after 11 years, he said: "My wife Cathy has been a huge influence throughout my life."
Many groups have sent messages of respect and condolences on their social media pages.