The detective hunting Holly Wells and Jessica Chapman has issued a dramatic personal appeal to their abductor: "Ring me by midnight tonight".

Police have set up a telephone hotline for the exclusive use of anyone who might be holding the ten-year-old Cambridgeshire girls to contact the officer leading the hunt, "one-to-one".

Details of how to contact Detective Superintendent David Beck have been sent to Jessica's Nokia blue 5110 mobile phone - not seen since the best friends vanished on Sunday last week - by voicemail and text message.

In a personal television and radio plea to an abductor, Detective Chief Inspector David Beck urges them to "work with me to stop this getting any worse than it is. You do have a way out".

Mr Beck did not spell out any consequences of missing the deadline.

In a move signalling police hopes that Jessica's missing phone may be in an abductor's hands, Mr Beck says in his message: "I have left you a personal message and a text message on Jessica's mobile phone."

Speaking slowly and emphasising certain words, he urges: "Listen to that message. It will tell you how to contact me so we can stop this now. You have an opportunity to speak to me. This is the time to use it."

Mr Beck also signalled that police believe the answer to the riddle of the girls' disappearance lies no further than their home town of Soham.

"The answer to this lies somewhere in this town centre," he said.

Detectives are playing down a potential sighting of the girls with a man in a green car on the way to Newmarket on the night they vanished after new evidence cast serious doubt on its timing.

A taxi driver insisted he saw the car at 7pm after picking up a fare nearby, but his front seat passenger has told police he made a mobile phone call as he got into the cab. That call was traced and was made at 6.01pm.

There are several sightings of the girls around Soham town centre after 6pm,

Wednesday night was a harrowing ordeal for the girls' parents, Nicola and Kevin Wells and Sharon and Leslie Chapman, as they waited to hear whether police had found their daughters in woodland shallow graves.

After an all-night excavation, detectives found nothing in the copse, near Newmarket, and believe the mounds of freshly-turned earth discovered by a jogger were nothing more than badger sets.

Last night, Mr Beck was waiting by a phone for the call he desperately hopes will come.

He told a news conference he had "legal and indeed a moral" duty to assume the girls were still alive, and did not care if statistically this was unlikely.

"I can't accept statistics," he said. "Records are there to be broken and, in the absence of any further evidence, I have to have that belief because the family has that belief."

Mr Beck said any call from a kidnapper would be connected to wherever he was.

He added: "If it is the case that they have been abducted - and we must assume there is a strong possibility they have been - then any dialogue we can initiate is going to be very important because we will then know the reasons for their disappearance."