The killers charge from pounds 1,000 to pounds 20,000 per murder and have been used by spurned lovers as well as gangs.
Scotland Yard has a special squad in operation to crack down on the hitmen, amid concern at the upsurge in the number of men prepared to kill for cash. David Veness, Scotland Yard's Assistant Commissioner in charge of specialist operations, said: "We have a genuine fear that there is a greater capacity for [criminals] to gain access to individuals willing to kill for money. There are worrying signs that there are small groups for whom this is the main form of criminal activity."
He said there was evidence that the London-based professional killers were being used to carry out hits in other parts of the country. Previous assassinations also suggest that hitmen are being brought in from abroad.
Mr Veness has a pool of 40 detectives from Scotland Yard's Organised Crime Group to help in the inquiry. At least 20 contracts have been carried out, although Mr Veness believes there are almost certainly many "hits" that have yet to be identified as the work of professional assassins.
Most of the killings are understood to be between rival crime gangs, particularly when in dispute over drugs. However, a number of cases have involved businessmen disposing of rivals or partners and of jilted lovers gaining revenge.
Mr Veness said: "The ones that concern us most are those where it's felt a criminal grouping can with relative impunity resolve its business disputes via killers. If that attitude develops, a general climate of criminal disorder rapidly follows." He added: "The other category of mistresses and distressed lovers does not create the same insidious effect."
Asked whether the hitmen are being hired for jobs in other parts of the country, he said: "We have examples of movement of activity within the UK."
Mr Veness said there was a "resurgence" in hits during the past five years because of greater availability of guns and possibly greater competition and rivalry between gangs. He estimated there were 10 to 20 contract killers operating in or from the Metropolitan police area (Greater London), being paid from pounds 1,000 to pounds 20,000 a hit. Detectives believe a significant number of unsolved murders are contract killings but so far no evidence has emerged linking them to hitmen.
In what is believed to be the latest example of a contract killing, a man was found shot dead in a wood in Highgate, north London, two weeks ago. Brandon Hale, 48, a known heavy gambler, was beaten and then shot above the left eye.
The Scotland Yard inquiry is hampered by the extreme secrecy surrounding the subject of contract killers, who usually have no connections with their victims.
Recent police successes against hitmen include conviction of a professional killer known as The Executioner, who was jailed for life in March for murdering a car dealer.
Kevin Lane, 26, was paid pounds 100,000 by an unknown underworld contact to shoot Robert Magill near his home in Rickmansworth, Hertfordshire, in October 1994.
Two men who were paid pounds 20,000 for the contract killing of the millionaire Donald Urquhart were jailed in February 1995.
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