More than a dozen of James’ family members, all dressed in black, walked behind the vintage Rolls-Royce hearse that her coffin arrived in.
The service began at 1pm at St Mary’s Church in Barnes, with readings and music from relatives.
James died at the age of 40 last month after a six-year battle with bowel cancer. She was known as Bowelbabe and spoke candidly about her experience with her illness and treatment, right up until her death.
Her fundraiser for Cancer Research UK collected more than £7m in donations, soaring past her initial goal of £250,000.
The private service in south-west London saw mourners carrying the sheet music for the song “Tell Me It’s Not True” from the musical Blood Brothers, along with a programme for the event with James’ photograph on it.
The song was sung by jazz singer Natalie Rushdie, wife of novelist Salman Rushdie’s son Zafar, during the service.
Her children, 14-year-old son Hugo Poland Bowen and 12-year-old daughter Eloise, both read poems at her funeral.
Hugo carried his mother’s wicker coffin together with his father, Sebastien Bowen, into the church and later read a poem he had written himself, while Eloise also delivered a piece.
The wicker coffin was adorned with a rose that bore James’ name. Ahead of the service, family members carried a wreath of white flowers into the church.
A note attached to the wreath said: “Deborah, thank you to our extraordinary friend. You together with Seb, Eloise and Hugo are such an important part of our family, we will love and miss you forever.”
The family came out of the church in silence and bowed their heads before leaving for a private wake, as Dame Deborah’s coffin was driven away from the church.
The mother-of-two and host of popular podcast You, Me and the Big C revealed that she was in hospice at home care in early May, telling her social media followers that “the time has come to say goodbye”.
In her final weeks, she was awarded a damehood by Prince William, who paid a special visit to her parents’ home in Woking, Surrey, for afternoon tea and champagne.
Her family announced her death on 28 June, sharing that she had died peacefully while surrounded by her family.
Additional reporting by PA