A private detective hired to investigate the disappearance of Maddie McCann before being convicted as a conman has been found dead at his Surrey mansion.

Kevin Halligen, 56, was taken ill last Monday at his Guildford home and subsequently died. Police said they were treating the death as unexplained.

There is no suggestion anyone else was involved in his death, reports Telegraph UK.

Halligen was employed by Kate and Gerry McCann in 2008 before being fired the following year after it emerged he had squandered much of the £500,000 (NZD $946,162) fee on "pointless tactics".

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He was accused of conning the fund to find Maddie McCann out of £300,000 (NZD $567,697). He featured in the 2014 Channel 5 documentary The McCanns and The Conman, during which he denied spending money from the family fund on first class travel and luxury hotel rooms.

In 2013, Halligen was sentenced to 41 months in a US prison in an unrelated fraud case after he pleaded guilty to defrauding Trafigura, a Netherlands-based commodities trading company, who hired him to free two of its executives who had been detained in Ivory Coast.

The detective was hired to search for Madeleine McCann after her disappearance in Portugal. Photo / Getty
The detective was hired to search for Madeleine McCann after her disappearance in Portugal. Photo / Getty

A court found he had squandered $2.1 million (NZD $3.9million) in expenses, including buying himself a luxury home. It is understood that Halligen had developed a drink problem.

A spokesman for Surrey Police said today: "We were called to an address in Cobbett Hill Road, Normandy, Guildford, on Monday following a report of a man in his 50s having been taken unwell, who subsequently died.

"The death is being treated as unexplained and a file will be passed to the coroner's office in due course."