They had a daughter named Mary Bing Jamie Alfreda Leonora Quick Kit age 33.
Actress Diana Quick, 61—who saved him from alcoholism—moved out of their Â£1.7million London mansion and ended their 27-year relationship.
A source said: 'This is a devastating split. Everyone thought their relationship was rock solid, but Diana wants nothing more to do with him.'
Friends believe Bill's packed workload after his Bafta-winning performance as ageing rocker Billy Mack in the 2003 film, Love Actually, is behind his split from Diana.
One said: 'Bill's career has taken off over the last few years, turning him into a huge star.
'But it's been a different story for Diana as it's harder for older actresses to find work. The pair have lived increasingly separate lives, and she's been left on her own.'
Pals now fear the 58-year-old actor may struggle to cope without Diana, who first found fame in TV's Brideshead Revisited. He called her his 'wife', though they never married, and she was seen as a stabilising influence.
Bill is a renowned workaholic and has already battled a string of booze and drug demons, previously admitting: 'I don't have a moderate muscle in my body. I used to drink for England, Ireland, Scotland and Wales.'
Our source added: 'Diana has been Bill's rock for years. He was a mess but got himself clean and became one of our most successful actors. One of the reasons was the support he got from Diana. They are saying publicly it is a mutual split, but the truth is that Diana got fed up with Bill and ended it.'
The couple met in 1981 and have a 24-year-old actress daughter Mary together. Bill once described Diana as the 'love story of my life'.
But recently she has spent more time at their Suffolk cottage, and is now understood to have found a new home. The source added: 'They're living apart and have cut all contact. It's tremendously sad, but it's all over.'
Last night, the couple said in a statement: 'Bill and Diana separated amicably. They remain great friends.'