Part-time worker Louise Allan, 41, says Mrs Blair asked her to take on more hours but the mum said she couldn't because she had to be home for her 14-year-old-son and nine-year-old daughter.
Miss Allain told the Daily Mail that she felt Mrs Blair had 'thrown her on the scrap heap' for wanting to look after her children.
Miss Allain said: "I feel an utter sense of injustice. It's sheer hypocrisy. How can she go from being a forward-thinking boss who you couldn't ask for more from to being so dispassionate and cold?
"She had turned her back on me because I would not work full time. She is supposed to empower women, she goes around the world empowering women, but when it comes to a single woman on her own and struggling in her own office, she just says get lost."
Miss Allain, who lives in central London, began working in the barrister's personal office just over a year ago. The two had met when her son was in the same class as Leo Blair.
When the single mother confided she was struggling to find work that fitted in with looking after her son and daughter, Mrs Blair offered her a post working 16-and-a-half hours a week.
For just over a year, Miss Allain worked at the office in London, replying to correspondence and updating Mrs Blair's website in her £12,500 job.
But after a reorganisation in January, she was asked to go full time, working until 6pm five days a week.
After the single mother wrote an email stating that she was unable to balance the extra hours, Mrs Blair replied: "I am really sorry Louise that it's not been possible to make it work but we need the role to be full time."
When Miss Allain asked if she was being fired, Mrs Blair responded: "You are being made redundant as there is no longer a part-time role."
Suggestions for job sharing or working from home were rejected, said Miss Allain.
Mrs Blair, who returned to work as a barrister after the births of each of her four children, has been a longstanding supporter of flexible working.
She has called for employers not to 'squander the talents of modern mothers'.
She has said: "We need to think about what we can do to support parents who want to prioritise full-time caring for a period when their children are young.
"We need to do more as individuals, as employers and as a community to make space for people to make these choices and rejoice in diversity in family life."
Commenting on Miss Allain's case, a spokesman for Mrs Blair told the Mail: "Cherie's office is very small, but she does try where possible to offer part-time jobs to women with young children, indeed there are two other part-timers with children who continue to work there."