Civil liberty groups want to ensure that details obtained during the mass DNA sampling are destroyed if they prove the person innocent. There is also concern that some people may have been named as suspects for malicious reasons.
The 120 men were identified by members of the public after the police issued computer enhanced photofits, or E-Fits, of two men who raped a 24-year-old woman in Regent's Park, London, in June. The incident provoked a strong public response. Detectives would normally contact each person identified as a possible rapist and take him to a police station for questioning. However, Scotland Yard said that with so many suspects DNA sampling would speed up the process.
The new tactic was revealed after the Metropolitan Police sent a three- page letter to 120 men inviting them to attend a London hospital later this month. They were told that a lawyer will be available for free consultation throughout the day and each man will receive pounds 10 expenses.
The letters say: "Someone has told police that you have an appearance similar to the published E-fits. That does not mean that you are the person we are looking for. In fact, by coming forward with confidence, knowing that it could not possibly be you, the inquiry team can then put their time and effort to where it is most needed and that is catching the person responsible ..."
A Scotland Yard spokesman said: "This may come as a surprise to most of the men but we would hope that they understand the importance of the exercise and help police to track down this rapist by eliminating a number of people from their inquiries." He added that information obtained from people who were later cleared would be destroyed.
This latest use of DNA sampling follows the establishment earlier this year of the world's first national databank of genetic profiles. Last week the police announced they had identified the first person - a suspected burglar - using a DNA sample from the database.
The civil rights group, Liberty, said: "We believe DNA sampling can be a very useful tool in the fight against crime. However, the police must ensure that information from suspects is destroyed if they are eliminated from the inquiries."
The technique used by police to build a facial image of the rapists uses the latest technology. The Scotland Yard team used E-fit computers into which all the details of a suspect are fed. The machine will produce a composite sketch, which can be altered by the witnesses.