• Despite police being called by a witness, they graded the call a 'Category Two' incident, meaning it only required a response within the next four hours

• Police were also given Riddall's numberplate but they only alerted their Automatic Numberplate Recognition cameras to the car hours later

A schoolgirl was raped after police failed to act on two phone calls from a member of the public who saw the victim being attacked, the Independent Police Complaints Commission (IPCC) has ruled.

Paedophile Colin Riddall pounced on the teen outside the gates of her school in Quedgeley, Gloucestershire as she walked home from a friend's house.

Riddall, 44, gagged and blindfolded his victim and drove her to a quiet lane where he beat her up.

The attack was witnessed by an ex-Metropolitan Police Officer who called 999 - but police at Gloucestershire Police graded the call a 'Category Two' incident, meaning it only required a response within the next four hours.

The former policeman followed Riddall's car and again phoned 999 less than fifteen minutes later, this time giving the fiend's registration.

But officers still graded the call a Category Two, despite Riddall's registration being linked on police computers to another suspicious incident just four months earlier.

An IPCC spokesman said yesterday: 'There was intelligence indicating that the vehicle had been seen in suspicious circumstances, apparently following a group of young girls in June 2010.

'This intelligence report was not viewed and the incident was therefore not re-graded.

'At 8.19pm the same evening, a 999 call was made to the police reporting that a 13-year-old girl had been kidnapped by a man in a white car and seriously sexually assaulted.

'It was later confirmed that this was the same vehicle and driver that had been reported in the two earlier 999 calls.'

Riddall - who had no previous convictions - snatched his 13-year-old victim on October 16 last year.

He was jailed indefinitely in February after admitting kidnap, false imprisonment, sexual assault and assault causing actual bodily harm.

Tim Beer, 54, the former policeman who witnessed the attack taking place said he had felt compelled to chase Riddall to get his car registration.

Mr Beer, who lives in Cornwall but was visiting relatives at the time of the offence, said: 'I was just having a drive in the country when I passed this car, parked up in Elmore.

'As I looked in I could see a female being frenziedly assaulted by a male. I couldn't see her face, so didn't know at the time how old she was.

'I parked up around the next corner, dialled 999 and told them what I'd seen. This was about 5.40pm.

'I thought I should try and get his number plate so told them I'd go back, which I did.

'But by this time, he'd gone. I sped around looking for him, but was certain I'd lost him, so I went back to Elmore.

'To my surprise, there he was again, parked up near the original spot.

'As I drove past he turned his face away from me, but I could see the girl looked terrified - her face was all red and crumpled as though she'd been crying.

'I turned around again, but as I got close to his car he put his foot down and sped off.

'I've been trained in pursuit, but we were doing 60mph down country lanes and it was getting silly.

'In the end, I memorized the number plate, gave up the chase, and called 999 again with the car's make and registration.

'I left it to the police to do their job. If I'd known this guy had a young girl in the car with him, I'd have forced him off the road without a second's thought. I do feel bad now that I let him go.'

Astonishingly, police only alerted their Automatic Numberplate Recognition cameras to Riddall's car after his victim called them hours later.

She was dumped half naked by the side of the road and stumbled to a friend's house where she raised the alarm.

Car valeter Riddall was spotted by police later that night in Wales after fleeing to Aberystwyth, and was arrested just after midnight.

Beads from his victim's top were found in his car.

And after he was confronted with CCTV images of him loitering outside the school gates he led detectives to the scene of his attack where they found his victim's phone and shoes.

Her mother said yesterday: 'She still suffers nightmares about what happened to her that night, and is terrified of going out alone.'

IPCC Commissioner Rebecca Marsh said: 'We will never know whether a much earlier response to the first 999 call would have been able to partially prevent this assault.

But it is clear that the errors in the initial grading of the first call meant that this incident did not receive the appropriate response and led to it being viewed with a misguided lack of urgency by those who subsequently dealt with it.'

The IPCC ruled the 999 calls were not dealt with efficiently and resources were not deployed to the incident, but said the calls made no reference to the victim being a child.

'The control room staff were clearly unaware of her age and vulnerability,' said a spokesman, adding that two inspectors, one sergeant and five control room staff were subject to performance advice for their individual inaction in handling the initial 999 call.