Many disabled victims of crime are being failed by the system, the Guardian has learned, following two cases in which the Crown Prosecution Service and the police were forced to apologise for not taking offences seriously enough.

Last month Gareth Williamson, 19, became the latest victim of crime to receive damages after the police accepted they were guilty of "serious failures" in their investigation of a repeat robbery of the teenager.

Williamson, who has mild learning difficulties, received ?70,000 including legal costs from Northamptonshire police after they did not adequately investigate the reported crime, despite the fact that it was caught on CCTV and confirmed by several witnesses.

His mother, Alison Williamson, said: "As soon as it was mentioned to the police that Gareth had special needs, it became obvious not much was going to be done. It's unbelievable the way he was treated ? we all felt totally failed."

Williamson, who was robbed twice within two months, said he was deeply affected by the failure to investigate his attack. "I was not believed because I had learning difficulties and as a result the people who attacked me have still not been brought to justice," he said.

In a letter sent on behalf of the chief constable, Peter Maddison, seen by the Guardian, the police admitted that they made "basic errors" in investigating the complaint and provided an "unacceptable" level of service.

The news comes as research by the mental health charity Mind shows that 60% of respondents in a survey did not feel they were taken seriously when they reported a crime.

Tony Murphy, a partner at Bhatt Murphy solicitors who represented Williamson, said: "It should not have required litigation for the police to honour its duties to people with disabilities in Northamptonshire. Sadly, I am dealing with these kinds of cases nationwide."

In another case in January, the high court awarded ?8,000 to a victim, known as "FB", after the case against a man accused of biting his ear off in a violent attack was dropped when evidence of his mental heath problems came to light. In a judgment which found FB had suffered a violation of his right against inhuman and degrading treatment, Lord Justice Toulson said: "The decision to terminate the prosecution ? was a humiliation. [It] understandably caused [FB] to feel that he was being treated as a second-class citizen."

The CPS later apologised to FB in a letter also seen by the Guardian. "I am very sorry that the case was dropped without a proper consideration of the issues and that you were left so distressed by our treatment of you," said Dru Sharpling, chief prosecutor for London.

Paul Farmer, chief executive of Mind, said that victims with mental health problems "face barriers at every stage of getting a crime brought to justice".