Ms Dando was shot on the doorstep of her home in Fulham, west London, last Monday, sparking an immediate murder inquiry.

But Scotland Yard refuted claims by the News of the World newspaper that police believe that the killer was a former male friend, who was jealous of her recent engagement. Ms Dando was going to marry gynaecologist Alan Farthing later this year.

The BBC's Tony Morris: "The motives still remain unclear with police investigating numerous lines of inquiry"

In response to the story, "Cops think Jill was killed by pal", the Yard's spokeswoman said: "We certainly have not put out that it (the killing) was one of this nature.

"But what we have said with regards to the motive is that we are keeping an open mind."

Jill Dando was due to marry her fiancé Alan Farthing later this year

The possibility of a second car, a silver BMW sportscar, being used in the murder was, however, ruled out by Scotland Yard on Saturday night.

The Sunday Times newspaper said investigations had been under way after a witness said they saw a man matching the killer's description acting suspiciously. He was then reported to drive away in a two-seater BMW Z3 minutes before the murder.

The sighting, twice in the area near Ms Dando's house, was reported in the newspaper, which named the witness. But Scotland Yard's spokeswoman said: "We would not name anyone we have spoken to or otherwise in connection to the investigation.

This is the first image of the man police want to quiz

"Our response to a second car being used, namely a silver BMW, is that at this stage, we do not consider a second car to be involved."

More than 1,000 members of the public have telephoned the police in connection with the murder, following Friday's release of a computer-generated picture of the prime suspect.

The BBC's Stephen Cape: Officers say the killer may have an accomplice

Scores of people also called the police after an appeal over a blue Range Rover seen speeding near Ms Dando's home.

And detectives are reportedly looking at a possible link with a similar hitman-style murder five years ago in Surrey.

But they admitted on Friday that they still had no firm evidence to link Monday's shooting to a clear motive.

There has been an increasing amount of speculation in the newspapers about a Balkan connection to the killing, particularly since it was confirmed that she had received a letter about the crisis.

Mr Campbell said: "I am not ruling out any line of inquiry. There's no intelligence that the murder relates to the Serbian and Kosovan crisis."