The chillingly formal letter lays out serious allegations against the popular cook and claims her actions have cost her former husband a fortune.
It warns that if she refuses to settle the matter voluntarily, legal action in court will ensue.
Saatchi, who was Miss Lawson's second husband, initially attempted to dismiss the shocking photographs that killed off his ten-year marriage as a "playful tiff".
He then went on to say "my actions were not violent" and that he was "disappointed" Miss Lawson had not publicly said he had "never abused her physically in any way".
Given that he has an estimated £100million fortune thanks to his successful career and extensive art collection, the sum of £500,000 involved in his dispute with Miss Lawson is negligible.
But a source close to Saatchi said: "Charles has been painted as the villain in all this - millions of people are convinced he was assaulting Nigella outside the restaurant.
"But there are very genuine reasons for why he was holding on to her.
"She has never given a public account of what was happening.
"He has just been damned by the photographs, her silence, and his acceptance of a police caution - which he did only to protect her from a lengthy probe by officers. But he has never been able to explain the truth about what was going on."
The couple's marriage ended in a brief hearing just seven weeks after the incident at Scott's. Miss Lawson, who has a £20million fortune of her own and two teenage children from her first marriage, had petitioned for divorce on the grounds of Saatchi's "unreasonable behaviour", saying the marriage had "irretrievably broken down".
The couple had married in 2003 following the death of Miss Lawson's first husband, broadcaster and journalist John Diamond, from throat cancer in 2001.
On a 'statement of truth' she signed as part of the separation from Saatchi, in reply to a question about how her husband's behaviour had affected her health she answered: 'Not applicable.'
Miss Lawson's spokesman last night refused to comment on the latest development in the saga. But a source close to the television chef and bestselling food writer said: "The seriously defamatory claims are, of course, denied."
Since their separation, Miss Lawson has been living in rented accommodation in Central London.
Saatchi, who lives in London, has been married three times. In recent months he has been doing his best to forget his marital woes by repeatedly dining out with glamorous journalist and television fashion expert Trinny Woodall - which has included repeated trips to Scott's restaurant.
- DAILY MAIL
