The source said a criminal profiler, brought in to examine the scene of the shooting, had cast doubt on one of the theories being examined - that she was killed by a crazed fan.

Jill Dando: 'Pure and innocent'

Ms Dando, 37, was killed outside her London home on 26 April with a single shot to her head.

After extensive examination of video footage, police have found no evidence that anyone had been following the presenter in the weeks leading up to her death.

The source said on Wednesday: "The shooting bears all the hallmarks of a very planned and precise homicide.

"That person was not there by accident.

"The person that day was in position with a 9mm firearm ready loaded and was there waiting for one single thing."

The photofit: Was this man seen running from the scene?

However, detectives investigating the case have not entirely ruled out the obsessed fan theory. There have been instances of people being killed by someone with a fixation on them even when no previous stalking has taken place.

Detectives are still looking into this possibility, and concentrating on what Ms Dando was doing in the months leading up to the murder.

The killer is believed to have been at the scene for only about 10 seconds before fleeing.

A forensic examination of the area around the front gate of Ms Dando's home in Gowan Avenue, Fulham, revealed only a single cartridge from a 9mm short handgun.

The source said there were no unsolved shootings in the London or Home Counties region where a similar gun was used.

The source said inquiries into Ms Dando's movements in the weeks before her death so far revealed her life had been "as pure and innocent" as her colleagues had told police.

He said investigations into patients of Ms Dando's fiancé, gynaecologist Alan Farthing, suggested none of them would have had a grudge against him that may have led to the killing.

Detectives are to investigate surveillance footage taken at Heathrow, Gatwick, Luton and Stansted airports to see if the killer fled the country after the shooting.

The source said he was "confident" that further investigations could still lead to an arrest.

Detectives also believe a man seen sprinting away shortly after the killing may have been a different man to the one seen sweating at a nearby bus stop and on whose face a widely circulated photofit picture was based.

The killing was highlighted on an edition of Crimewatch UK - which Ms Dando used to present.

But, despite the programme leading to more than 500 calls from the public, no important new leads had been thrown up, Scotland Yard said.

Clues to a murder