The man convicted yesterday of murdering two women and trying to kill a third was left free to continue his campaign of violence against women because of basic investigative errors by police.

Levi Bellfield was found guilty at the Old Bailey of murdering Marsha McDonnell, 19, and Amelie Delagrange, 22, and attempting to murder Kate Sheedy. He was then identified as the prime suspect for the unsolved murder of the schoolgirl Milly Dowler.

A Scotland Yard task force has been set up to investigate Bellfield’s possible connection to a series of 20 murders, attempted murders and other attacks dating back 25 years.

Detective Chief Inspector Colin Sutton said that Bellfield was a dangerous man and that women would be safer now that he was behind bars. He stalked his victims, all young and blonde, as they waited for — or stepped off — buses late at night.

Police said that they expected more victims of Bellfield, 39, a wheelclamper, to come forward after his picture was published widely for the first time.

As officers promised to uncover the full extent of Bellfield’s crimes, it became clear that vital clues were missed that could have led to his arrest two years before he was eventually detained.

Four officers from the Metropolitan Police have been reprimanded for serious errors in the inquiry into the attempted murder of Kate Sheedy in Isleworth in May 2004, three months before Miss Delagrange’s death.

Bellafield had attacked Miss Sheedy, now 21, with his car, knocking her down then driving over her twice.

The Independent Police Complaints Commission found that officers had looked at the wrong day’s CCTV footage, thereby failing to spot Bellfield’s car stalking Miss Sheedy.

The Times has learnt that the written warning given to one officer was considered so serious that it was handed down by a Deputy Assistant Commissioner at Scotland Yard.

Police in Surrey investigating Milly Dowler’s death failed to follow up on house-to-house inquries that could have led them to Bellfield in 2002 — before the murders of Miss McDonnell in 2003 and Miss Delagrange in 2004.

They called 11 times at the house of Emma Mills, Bellfield’s girlfriend, but after being told that she had moved they did not trace her new address. Miss Mills was by then living with Bellfield in West Drayton, West London.

She has since been able to tell police that she owned a red Daewoo car of the kind seen on CCTV near Walton station on the day that Milly went missing. The car was reported stolen four days later.

It has also emerged that a man in a similar car attempted to abduct a girl in the area the day before Milly disappeared.

A previous girlfriend of Bellfield’s had a daughter who was a schoolfriend of Milly. The schoolgirl had been to her classmate’s for tea and was thought to have met Bellfield.

Miss Mills is also understood to have told detectives that, on the night of Milly’s disappearance, Bellfield got out of bed at 4am and told her that he was going “to take care of the dog” at her flat in Walton. Detectives think this might have been when Bellfield went to dispose of Milly’s body.

Her remains were found in September 2002 in woods at Yateley Heath, Hampshire, an area known to Bellfield, who attended car auctions nearby.

Surrey police have issued a fresh appeal for information about the Daewoo car and are offering a £50,000 reward for evidence leading to the conviction of Milly’s murderer.

Bob and Sally Dowler, Milly’s parents, appealed for new witnesses to come forward. They said: “Milly was a loved and loving daughter and sister. She had every right to expect a happy, rich life ahead of her. As parents, how could we imagine anything else? We are pleading for anyone who knows anything to have the courage to speak up.”

Surrey police sources emphasised that nothing had emerged in 2002 to make Bellfield a suspect. At the time he had nine previous convictions and there was intelligence to suggest that he made silent phone calls to women. “He was way off the radar, it was not until he was picked up in connection with the Amelie Delagrange murder that he began to be a possible suspect,” said one source.

Bellfield was caught eventually by a combination of painstaking detective work and a bizarre stroke of luck.

A police telephone hotline set up after the murder of Miss Delagrange received 129 calls from women who suspected that men they knew might be the killer. One woman said that Bellfield, who had violently attacked her during their relationship, was capable of killing women.

At the same time officers were studying CCTV pictures from the streets around Twickenham Green on the night of the young French woman’s murder. They appeared to show that she was followed by a white Ford Courier van but were too blurred to yield the vehicle's numberplate or driver. A smudge on the roof and two aluminium rear plates were the only things to mark it out from 26,000 similar vehicles in Britain.

The search for the van produced information about a wheelclamper who had bought a similar van for cash a few months earlier. The man had left his mobile phone number with the vehicle’s previous owner.

Then came the stroke of luck. When the phone number was punched into the police intelligence system, it matched with the number of a man who had called the antiterrorist hotline months earlier to report suspicions about his neighbour.

The man was Levi Bellfield — the same name given by the woman caller. Surveillance revealed that his white van had similar markings to the one caught on CCTV.

His phone had also rung moments before Miss Delagrange was attacked. Bellfield, whose awareness of forensic science meant that he normally had his phone off before attacking his victims, immediately switched it off. But both the call and the act of switching the phone off left traces on the network that placed Bellfield at the murder scene.

When police raided his home to arrest him in November 2004 they found him cowering in the loft.

The jury was unable to reach verdicts in connection with two other attacks, on Anna-Maria Rennie, then 17, in Whitton, South West London, in October 2001, and Irma Dragoshi, then 33, in December 2003. There was no evidence that any of his victims had been sexually assaulted.

Bellfield will be sentenced today.

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