Devon and Cornwall Police failed to investigate more than 30,000 crimes last year – more than one in three of all offences – after ruling that there was little chance of catching those responsible.

Crimes disregarded by the force included more than 10,000 incidents of criminal damage, more than 3,000 thefts from vehicles, more than 1,000 assaults, and hundreds of sexual offences.

It meant that of 87,095 crimes recorded last year, 30,511 – or 35 per cent – were marked "not for further investigation" after an initial assessment.

One Westcountry MP said it was a concern that such an "exceedingly large number of crimes are consigned to oblivion at a stage where no investigation has been conducted at all".

Figures released by the force under the Freedom of Information Act showed that the number of offences "screened out" had risen by 12 per cent since 2008. Police were least likely to investigate "vehicle interference", where 282 offences (63.9 per cent) were committed but not investigated.

That was followed by "theft from vehicle" with 3,244 offences (60.2 per cent), "other theft" with 7,868 (57.3 per cent) and criminal damage with 10,791 (56.4 per cent).

But the table also showed that 1,685 drug offences (32.7 per cent) went without inquiry as did 362 sexual offences (21.4 per cent) and 1,824 (28.6 per cent) incidents of shoplifting.

Torridge and West Devon MP Geoffrey Cox conceded that there were times when trying to investigate certain crimes would be a "wild goose chase". But he said there was "a victim behind every figure", and raised particular concerns over the rate of the sex offences that had been ignored.

"It is a clearly common complaint from time to time from constituents that the police tell them there is no practical point in any investigation," the Conservative MP said.

"It always leaves people with a sense of helplessness and frustration, and doesn't do the reputation of the police or the criminal justice system as a whole any good.

"I hope the police are scrutinising these cases to ensure that they are being properly categorised, because there is nothing more frustrating for a member of the public to be told that the police aren't even bothering to investigate their complaint."

Mr Cox said there had to be "very rigorous criteria and sufficient quality control" to ensure that crimes were not being wrongly dismissed.

"It does beg the question whether in borderline cases, particularly at a time of financial constraint, the police are devoting the necessary time and effort to assess whether there could be a favourable outcome," he said.

A force spokesman said an improvement in "recording and processes" explained the 12 per cent rise in offences being disregarded between 2008 and 2010. He said the figure was expected to rise again in 2011 as further improvements were made.

In its Freedom of Information Act response, the force said "investigation of a reported crime starts immediately within our call handling centre.

"Those crimes, where there is viable potential for a positive outcome will be allocated for further investigation.

"Where there is little potential for such an outcome, victims will be informed at the earliest opportunity and offered crime prevention advice and the services of Victim Support.

"However, this is not to indicate that there will be no subsequent investigation if further information comes available. Similarly, as part of our crime pattern analysis we include all crimes to identify crime patterns and series. This research can lead to identification of offenders.

"Assessments of crimes for further investigation are conducted by a trained member of staff who looks at each crime to ensure that all investigative opportunities have been explored and whether further investigation is required."

The force said it continually reviewed its investigation processes to ensure that it met the needs of the public.

"Effective crime assessment is essential as it allows us to prioritise our resources to ensure we deliver the best possible service to the victims of crime," it said.