Research by Mencap showed the number of cases of abuse against adults with learning disabilities was much higher than the 1,400 a year estimated by the government- and much abuse occurred in care.

Speaking on BBC Radio 4's File On 4, Mencap director David Congdon said only a quarter of cases were investigated by the police, only 6% of alleged perpetrators were prosecuted, and less than 1% were convicted.

David Congdon: Wants laws tightened

Mr Congdon, who served on a Home Office working party on sexual offence laws, said: "Professionals who deal with abuse every day had the very, very strong view that child abusers are now moving on to adults because they know they can get away with it."

"Very few cases get to court and even if they do, very few get a conviction.

"Offenders recognise they stand a good chance of being prosecuted and spending time in jail if they abuse children. They know the public are horrified by it.

"But we've seen some notorious cases thrown out of court because the judges have decided that a person with a learning disability is not a credible witness.

Issue of consent

"It's an outrage for them. Their parents get very, very angry because they do not get their day in court to present their case."

Mencap said this gap in the law needs to be remedied to root out abuse.

A lot of people with learning disabilities are dependant on a care worker for toileting, washing, etc, and that does offer opportunities for abuse

David Congdon Mencap director

The maximum prison sentence for someone found guilty of having sexual intercourse with "a woman who is a mental defective" is just two years. Mencap want the sentence extended to life.

Not only is the term "mental defective" offensive, said Mr Congdon, but there needs to be a clearer definition of incapacity to give consent.

"It's generally difficult to get a prosecution over rape because of the issue of consent.

Allegations over carers

"It's even more difficult if the person has a learning disability because of communication problems - and sometimes because of the actual capacity to understand the nature of the sexual act."

Mencap campaigners also want to see better definition of an offence of an abuse of trust in any care setting to put off potential offenders.

Mr Congdon said: "A lot of people with learning disabilities are dependant on a care worker for toileting, washing, etc, and that does offer opportunities for abuse.

"If a person engages in sexual activity with someone in their care, all you should have to prove to make it an absolute offence is that the event took place - which is clearly still difficult but a lot easier than having to prove consent.

"If you did get a few convictions the message would be very, very firm to potential abusers."

File on 4's report on the growing concern over the sexual abuse of adults with learning disabilities is broadcast at 2000 BST on Tuesday 11 June on BBC Radio 4, and on the File on 4 website.