Police have been accused of failing to act twice after childcare teams reported a teenage girl was being groomed for sex by an older man.

It was only when Brighton and Hove's social services team threatened to make an official complaint that the Sussex force pressed charges.

The disturbing case is one of a series exposed following an undercover investigation for BBC1's Real Story, due to be screened tonight.

Investigators believe many social services protection teams and police officers are confused about the law and fail to take action against older men who entice under-age teenagers into having sex.

Young girls who have been groomed by older men are often reluctant to give evidence against men they consider their boyfriends.

Ignoring serious allegations made by parents or relatives, half of the 50 authorities contacted were under the mistaken impression they needed a direct complaint from the victim before they could investigate.

Colin Tucker, Brighton and Hove's assistant director for children, families and schools, said he could not comment on specific cases. He said police officers were sometimes opposed to pressing charges due to lack of evidence.

He insisted they should persevere with an arrest because it sent out a message that under-age sex would not be tolerated.

Mr Tucker said: "We would not use the term paedophile when referring to these offenders because it is misleading.

"The victim can be a 14-year-old with an 18-year-old man and the sex is consensual.

"I would negotiate with the police on whether we think a person should be prosecuted or not.

"My view is, while these things have to be taken on a case-by-case basis, sometimes we should prosecute because the message we give to young people is important.

"Evidence or not, it's important to take formal action.

"This BBC investigation does raise the fact that, nationally, this is a grey area and there are probably inconsistent practices.

"In Brighton and Hove, the reason I am able to negotiate and discuss cases confidently with the police is because we have a very good grasp of the law and use the law to protect children.

"There have been cases where the police have refused to prosecute due to lack of evidence, not lack of will.

"Sometimes the right thing to do is to go as far as we can with the minimum of evidence. Getting the man arrested even though we do not have enough evidence to prosecute is important."

Chief Constable Terry Grange, the Association of Chief Police Officers' child protection spokesman, said many forces did not properly investigate allegations.

He said: "In law, we're talking about an adult raping a child and, whether or not the child knows it's rape, we should investigate."

Mr Grange advised parents to "kick up a fuss" if their child was targeted by an older man.

Real Story is at 7.30pm.