The star witness in the case against former police inspector David Gatt yesterday confirmed he was arrested and investigated over an allegation he slept with a prostitute while on duty.

Police Constable Mario Portelli, who has become known by his number PC 99, was testifying against Dr Gatt, now a lawyer.

He had previously testified that Dr Gatt had masterminded a string of high profile hold-ups, over which he is facing charges, including the failed heist on the HSBC headquarters in June 2010 and another unsuccessful robbery on an Attard jeweller in November that year.

PC Portelli confirmed under cross examination by Joe Giglio that he had been arrested and investigated during the case, saying Dr Gatt had told the police that he, Mr Portelli, had sex with a prostitute while on duty.

He gave no further details.

Dr Giglio asked why Mr Portelli should wear jeans in court, especially as the sitting was planned in advance and he was still a serving officer. The officer said it was his day off. Dr Giglio said it was inappropriate that PC Portelli should wear jeans, deeming it disrespectful to the court. No action was taken.