FOR years, Agatha Christie fans have puzzled over the one mystery she never solved, but now a Poole biographer claims to have the answer.

In 1926, the crime writer vanished from her Berkshire home for 11 days, leaving no word with family or friends.

Theories have abounded over the strange incident for 80 years, but Dr Andrew Norman, a former Poole GP who turned to writing following a back injury in 1983, says a little-known psychological state holds the key.

"During those 11 days, she entered what is known as a fugue state' - a sort of out-of-body trance - that was caused by tremendous mental trauma, and left her unable even to recognise her own picture in the paper," he says.

Little understood at the time, this "psychogenic amnesia" may also have led writer and actor Stephen Fry to travel unannounced to Bruges for several weeks in 1995, Dr Norman believes.

Dr Norman makes the case in his new book - The Finished Portrait - released last week.

Christie, who already had six popular novels in the shops, kissed her daughter goodnight on December 3 and walked out of the house.

Her abandoned Morris Cowley was later found by a quarry near Guildford.

The following investigation was widely reported, even drawing in Sherlock Holmes creator Arthur Conan-Doyle and the attention of the Home Office.

She was eventually tracked down in Harrogate, living in a spa hotel under an assumed name, and unable to explain her actions.

Dr Norman said: "She was probably suicidal. The loss of her mother and knowledge her husband was being unfaithful drove her into this state. People have said it was amnesia caused by the car accident, but she wasn't asking anyone for help.

"She wasn't frantic, she was quite purposeful.

"Instead, she invented a back story for herself as Teresa Neale from Cape Town. She joined in at the hotel, talking to guests and playing piano. She was bemused, but not scared. It was like a trance."

Dr Norman got his first clue from an autobiographical novel, Unfinished Portrait, written under a pseudonym, and describing the deep depression she was suffering at the time, and later attempts at suicide.

"Many aspects in the book actually happened in her life, including meetings with doctors trying to explain these incidents.

"The symptoms all point to this fugue state."

Dr Norman believes his book finally puts paid to the theories that it was a publicity stunt or attempt to thwart her husband's infidelities.