Cynthia Crawford also revealed that her boss, whose energy and need for just a few hours of sleep a night is legendary, used to have vitamin B12 injections in her buttocks to sustain her.

Mrs Crawford, known as "Crawfie" to Lady Thatcher, makes the revelations in a documentary to be screened next month.

She recalls a night of drinking in Paris in November 1990 when Mrs Thatcher learned that she had secured only a lacklustre win in the first round of the party leadership contest which was to end her premiership.

Mrs Crawford said: "She came back from a dinner worn out. But then we sat up all night drinking. She spoke about her whole life: childhood, her father, Denis, the twins, everything. We never went to bed."

During the Falklands crisis, the Iron Lady also turned to drink, according to Mrs Crawford.

"We would sit on her floor at night. By the end of the Falklands, I'd become hooked on whisky and soda."

A "snifter" of gin and tonic was always a favourite of Denis Thatcher's. But Mrs Crawford said: "I remember her once saying to me: 'Dear, you cannot drink gin and tonic in the middle of the night. You must have whisky to give you energy."

Bell's is said to have been Mrs Thatcher's favourite brand.

Mrs Crawford also said the prime minister was given injections of B12, the structure of which was discovered by a teacher of Lady Thatcher's at Oxford, Dorothy Hodgkin.

The vitamin is important in the production of the genetic material in cells and of red blood cells in bone marrow, and in the utilisation of folic acid.

According to her former aide, Mrs Thatcher had a shot of the vitamin on the morning she told her cabinet she was going to resign.

Other lesser-known characteristics of the former prime minster are highlighted in the documentary.

Her faith, for instance, is illustrated in an anecdote about the aftermath of the Brighton bomb. Prime minister and aide were spending the night at a police training college. Mrs Crawford said: "Margaret told me to kneel beside her and pray."

The documentary, which sets out to be a personal rather than political portrait, also suggests Mrs Thatcher's appearance was very important to her. Her former political rival Shirley Williams recalls: "Whenever I went into the ladies' room at the house, there was Margaret ironing a dress."

A story in a biography written by Brenda Maddox, which accompanies the series, describes how Mrs Thatcher once lent Sue MacGregor her Carmen rollers when the former Today presenter was having a bad hair day during a foreign trip.

Anne Lapping, the executive producer of the series, made by the company Brook Lapping for ITV, yesterday said the revelations about Lady Thatcher's drinking and the injections explained how she had managed to keep going at times of huge pressure.

She said: "What surprised me most was how we have forgotten what an amazing achievement her career was, how she had to fight sex and class discrimination."