Almost half of all cases handled by top divorce lawyers last year involved a private detective to check on suspicions that a spouse was cheating.

Research found that demand for private investigators has rocketed, with 49 per cent of the affluent spouses paying up to £200 an hour for their services in 2006 compared with 18 per cent the year before.

Wives were far more likely to call in an investigator than were husband, a survey of 100 top divorce lawyers found.

In 30 per cent of divorce cases, women employed a professional to check up on their husband compared with just 19 per cent of men who checked on wives. The findings form part of the Grant Thornton Forensic and Investigation Services annual survey of 100 leading family lawyers.

It found that affairs were on the rise as the main cause of marital breakdown, with adultery cited in 32 per cent of cases, up from 29 per cent in 2005.

Andrea McLaren, head of the Grant Thornton matrimonial practice in London, said: “For the fourth year running, our survey has shown that extra-marital affairs are the primary reason cited for the breakdown of marriages.

“As this figure continues to rise, it is little wonder that the number of individuals using private investigators continues to rise.

“While we anticipated that this figure would increase, we had no idea that it would be such a significant increase.”

Martin Tomlins-Young, a director of Research Associates, was less surprised. Business was booming, and more than half his cases were matrimonial, he told The Times: “When you have spent a large tranche of your life with one person, you want to know for certain that your suspicions are correct. No one wants to throw away a marriage on a hunch.”

In about 90 per cent of cases, suspicions were proved correct, he said.

Despite technological advances, trudging the streets and staking out hotels and office blocks was the mainstay of any investigation. Teams of up to four investigators at a time were enlisted to monitor a suspect’s movements, and many used motorbikes to beat the traffic. “All our agents carry digital equipment, cameras and tape recorders, although there are tight regulations on where we can record,” said Mr Tomlins-Young.

Wives have been known to disable their husband’s car, forcing them to borrow theirs, which has been fitted with a recording device.

Mr Tomlins-Young said that every case was different. Some clients needed only to know that their spouse was not where they said they were to be convinced of an infidelity. Others needed photographic evidence of infidelity before they were satisfied.

And it can often be impossible to prove. Ingrid Tarrant, the wife of Chris Tarrant, the TV broadcaster, hired a private detective when she became suspicious of her husband’s behaviour.

The detective could find no evidence of a lengthy affair, although the presenter of Who wants to be a Millionaire? later confessed.

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