Once it was the murky world of dirty raincoats and skulking in shadows. But the private investigator is now fast becoming a standard aid to divorce.

Paul Hawkes, 49, has run his own firm, Research Associates, in West London, for 31 years. “Last year I had probably 100 to 200 cases involving checking on extra-marital affairs,” he said. “Ten years ago it would have been fewer 50.”

One reason for the change was that women in particular, who were the bulk of the clients, were now far more “pragmatic” and “not prepared to suffer in silence or sweep things under the bed. Now they want to know what is going on.”

The image of the private investigations business had also changed, he added. “There was a perception that it was about dirty macs and dirty dealing . . . but we are not allowed to do anything illegal.”

The laws, such as the Regulation of Investigatory Powers Act 2000, had ensured that surveillance activities were far greater controlled; and bodies such as the Association of British Investigators were lobbying for tougher regulation, he said.

The availability of credit cards also helped women to pay for such services, he added. “We use a shadow name so that it does not come up on the credit card.”

Mr Hawkes said that in the past, private investigators would be used by jealous husband or wives who had no basis for a suspicion. “But we don’t do that kind of work now. The people who use us want third -party proof. They suspect something is not right and they can see changes in behaviour, and they just want confirmation.”

Husbands, he said, might suddenly be uncontactable at certain times, or be “attatched to their mobile, even when they are in the shower” or start deleting all their text messages on their mobile phones.

He said a typical job, for which he charges £55 an hour, might last three days but could be as swift as one evening’s work. “We try to be proportionate: just to do what we need to do to get the information."

He uses ex-army surveillance operatives and other professionals on jobs. Tactics involve placing recording devices on people’s phones (which was allowed at the request of the wife or husband); tagging devices to track people’s movements, either on foot or by car; and taking copies of their computer hard drive, which can then be examined.

The use of private investigators was no longer regarded as something shameful, he added, but as a lawful and respectable means to establish the truth.

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