A statement released by the family read: "We are heartbroken to announce that our amazing mum Janice passed away today after an ongoing battle with cancer.
"She was an inspiration to us all and was loved by everyone who knew her. She died at Guy's Cancer Unit, London Bridge Hospital, with her loving husband Peter and us, her four children, at her bedside.
"She was a devoted mother to us all, and she was the most incredible grandmother to our children.
"The whole family are devastated. Our mum was the centre and the heart of our family and has supported us all through many difficult times, always with a smile on her face. Her strength and courage, as well as her great sense of humour and immense kindness, were never-ending.
"We would like to thank all the staff at Guy's Cancer Unit for looking after our mum so well during her illness especially Marta and Joanna as well as everyone who has sent kind wishes to us all during this difficult time."
United later tweeted a message of condolence, posting: "Our deepest sympathies go to Rio Ferdinand and his family at this sad time."
Mrs St Fort met Julian Ferdinand, Rio and Anton's father, as a teenager and they lived together in south London before the relationship ended.
She later married Peter St Fort and had two further children, Sian and Jeremiah.
Former England international Rio lost his wife, Rebecca Ellison, in 2015 after a battle with breast cancer.
His mother featured in the short film 'Being Mum and Dad', a documentary in which he met other young widowers and offered insight into how he and his three children have coped with their grief.