Anthony Diedrick, 39, told an Old Bailey jury that although he had secretly listened at the gynaecologist's bedroom window at night, talked to friends about her and tried to contact her through his mother, it was not an obsession.

"I did not set out to do it," he said. "I can't explain why I did it."

An artists impression of Mr Diedrick in court

Mr Diedrick, a computer operator of Maida Vale, west London, denies killing the doctor, who was found with a vacuum cleaner flex around her neck in her north London flat.

The doctor's body was discovered on Boxing Day 1994 - six years after a relationship between she and Mr Diedrick had ended.

His DNA was found in three tiny specks of blood on the dark pink T-shirt Dr Francisco was wearing.

The prosecution alleges Mr Diedrick became obsessed with Dr Francisco, 27, after they broke up and rekindled his obsession a few months before her death.

He is alleged to have killed her when he confronted her as she prepared to leave for the USA.

The doctor's body was found at her north London flat

When asked by Nigel Sweeney, prosecuting, how his blood, which experts said would not survive washing, got there, Mr Diedrick said it must have been before 1989.

"It must be because I did not murder Joan," he said. "I could not murder Joan."

Mr Sweeney asked: "Are we to take it that this much-washed T-shirt somehow remained with your DNA over six years?"

Mr Diedrick replied: "There must be an explanation."

Earlier, he admitted smashing into Dr Francisco's family home in Acton, west London, in February 1989, through patio doors and threatening to kill her new boyfriend.

The case continues.