Hundreds of ex-cons will be getting a knock on the door from detectives in a DNA collection drive to crack unsolved crimes.

The criminals are subject to a new police power to take a genetic sample from anyone convicted of a crime, even several years ago.

Sussex Police and other forces are visiting serious criminals who were jailed before the national DNA database was invented.

They hope that when the samples are added and automatically cross-checked against crime scenes and wanted appeals, they will find the culprits to unsolved crimes.

Nationally about 12,000 criminals, many of them murderers and rapists, are being targeted. In Sussex the figure is said to be in the “low hundreds”.

Visits have already started and are expected to continue until next spring in what has been dubbed Operation Nutmeg.

A spokesman for Sussex Police said: “DNA evidence has proved to be a vital tool which has helped convict thousands of violent and dangerous criminals and exonerated many innocent people. We are working to ensure that all lawful steps are taken to ensure that no opportunities have been missed to secure justice for victims of crime.”

DNA has unlocked several unsolved cases in recent years. In June Rupert Crawford was jailed for 27 years for two rapes in 1985. Sussex Police’s cold case investigators linked his DNA to material found in a house where he broke in and attacked a woman.

On Tuesday, Graham Wood, from Rushden, Northamptonshire, is due to be sentenced for the rape of a woman in an alleyway in The Goffs, Eastbourne, in April 1987. He too was caught after DNA was re-examined.

Amanda Cooper from Thames Valley Police, who chairs the police’s DNA Strategy Board, said cases would go back to the establishment of the police national computer, which first came into use in the 1970s.

She said: ''This isn't just about the retrospective matches, it's about crimes going forward.''

Offenders can provide a sample on the spot or arrange to visit a police station. They can be forced to provide a mouth swab if they try to refuse to take part.