The Telegraph reported that the 56-year-old fell ill at a home in Surrey, south-east England, last week and police are treating the death as 'unexplained.'
A Surrey police spokesman said, '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.'
According to reports Halligen was employed by Kate and Gerry McCann in 2008 to investigate the now infamous of their missing daughter Madeleine, who disappeared from the family's holiday apartment in Portugal on May 3, 2007.
Halligen was later accused of conning the Madeleine McCann fun out of £300,000 before his contract was terminated.
He was later sentenced to 41 months in a Us prison in 2013 for unrelated fraud charges.