IT WAS the most enduring mystery of her life, but Agatha Christie never spoke of the "disappearance"that caused a sensation in 1926. Friends and family knew not even to allude to it, and Agatha made no mention of it in her posthumously published autobiography 50 years later.

There was nationwide interest when the writer vanished after apparently crashing her car near a Surrey chalk pit. Once she was discovered, the speculation as to why she went missing continued for much longer.

As far as the Christie family was concerned, the answer was simple - her then husband, Archie Christie blamed his wife's disappearance on "an unquestionably genuine case of amnesia", and nothing more was to be said about the incident. The idea was that keeping quiet would deflect publicity; the family has stuck to the same explanation ever since.

However, it has been revealed that far from losing her memory, Agatha knew exactly what she was doing on December 3, 1926.

In his book, Agatha Christie and the Eleven Missing Days, published in 1998, Jared Cade reveals how the author staged her disappearance with the help of a co-conspirator, her friend and sister-in-law Nan Watts.

Agatha had indeed vanished at a time of severe mental torture - earlier that year her beloved mother had died and then Archie dropped the bombshell that he didn't love her and wanted a divorce so he could marry Nancy Neele.

Realising her marriage was over, Agatha, together with Nan Watts, staged the disappearance, deliberately making it look suspicious so the finger of blame would be pointed at Archie. The idea was to make him stew for a few days while the police questioned him.

Speaking about the saga for the first time, Nan's daughter, Judith Gardner, has revealed what really happened - and has confirmed that the family was never allowed to speak about it to Agatha.

Now 84 and living in Paignton in Devon, Mrs Gardner says it was the most traumatic period of the writer's life. "Of course, none of the family ever spoke of it to her because we knew it would upset her. She loved Archie very much and the whole thing was terribly upsetting for her. As far as she was concerned, once it had happened it was a closed book."

Mrs Gardner and her husband Graham are the only surviving members of Agatha's family from the time, other than her daughter Rosalind Hicks who still lives at Greenway, Agatha's last home outside Torquay.

The Gardners appear tonight in a documentary, based on Mr Cade's book. The BBC Leeds Close Up North series filmed Agatha: the Final Mystery on location in Berkshire, Surrey and North Yorkshire.

Agatha and Nan had been friends since they were young girls, having been introduced by their mothers who were school pals. They remained friends for many years and when Agatha's eldest sister Madge Miller married Nan's brother James Watts they became family.

Agatha was treated as an aunt by Judith and was a regular visitor to the Watts family home, Abney Hall in Cheshire.

"Agatha and mother were very close," says Judith. "Every time Agatha wrote a new book she'd send mother a copy with a dedication written inside. No one else would understand what the messages meant, they were personal to the two friends."

When Agatha was in emotional turmoil over her mother's death and Archie's request for a divorce, it was Nan to whom she turned.

On the morning of December 3, 1926, Agatha and Archie had a fierce row; Mr Cade says Agatha and Nan got together that afternoon to hatch their plan.

At about 9.45pm, Agatha got into her Morris Cowley car at her Berkshire home, Styles, and drove off into the night. Five miles outside Guildford in Surrey, at a beauty spot known as Newlands Corner, her car went off the road. It was discovered the following morning and the police were called. On the back seat of the car lay a fur coat, a case of clothes and Agatha's driving licence.

A full-scale search of woods in the area began while frogmen dragged a nearby pond. Thousands of volunteers and Archie joined in the hunt. When her body could not be found, the net was widened but by this time Agatha was already on her way to Harrogate.

After abandoning the car - leaving the items on the back seat as clues - she had caught a train to Waterloo, arriving later at Nan's house, 78 Chelsea Park Gardens in London. The friends spent the next morning shopping for clothes at the Army and Navy Stores and Nan withdrew some cash from her Harrods bank account. Agatha posted a letter to Archie's brother saying she was going to an un-named spa in Yorkshire and then caught the 1.40pm train north.

Once in Harrogate, she booked herself into the Hydro hotel (now the Old Swan) under the name of Teresa Neele - the surname chosen because it was the last name of her husband's lover Nancy.

She spent the next ten days writing, knitting, doing crosswords, reading and dancing at the hotel, and visiting the town's library. In fact it was her love of dancing that eventually revealed the truth about who she was - bandsman Bob Leeming became suspicious about her identity and, on December 13, the police were brought in.

Where the plan went wrong was that the two women had underestimated the amount of attention the case would attract. As someone who didn't like press coverage, Agatha was horrified at the national publicity that followed.

"Neither of them had any idea what would happen," says Mrs Gardner. "In those days the media didn't get so excited about famous people. For people to suggest Agatha did it for publicity is ridiculous.

"She certainly wasn't shy as some people have claimed, she enjoyed life very much: tennis, picnics, food, theatre and opera. She was a forthcoming, very loving, moral person but she didn't like noise in the sense of parties and publicity."

However, Mrs Gardner does admit that Agatha was walking an emotional tightrope in December 1926. "The two things coming together in those days would have been enough to make anyone go off their head. Divorce was a very serious matter, a terrible stigma and with a child to look after and not a great deal of money at that time, Agatha was distraught. Mother was her standby."

Agatha and Nan remained friends and confidantes into old age; Graham Gardner reveals how Agatha visited his mother-in-law several times during the last week of her life.

The Gardners say they chose to speak out following the recently re-released film Agatha - a purely fictional account of what went on in Harrogate starring Dustin Hoffman and Vanessa Redgrave - and because they wanted to end the speculation about the disappearance once and for all, even though Judith knows the Christie family wish the amnesia explanation to stand. Agatha's daughter, Rosalind Hicks, and her grandson Mathew Pritchard have declined to co-operate with the programme.

Mrs Gardner says: "We felt before we died that we should talk about what we know, to say Agatha wasn't in an unbalanced mental state other than to be very distraught and that she certainly didn't to it for publicity."

Mr Cade, who spent six years researching the story, says the Gardners are impeccable sources. "Nan Watts married into the family in 1902 and she knew Agatha well. The truth is that Agatha's disappearance was planned and carried out with the help of her sister-in-law."

l Agatha: The Final Mystery is in on BBC2 tonight at 7.30pm.

Agatha Christie and the Eleven Missing Days by Jared Cade (Peter Owen, £18.95)