G4S hit by connection to Orlando shooter Omar Mateen

Mateen, who has been identified by state police as the gunman, was confirmed by G4S as an employee since September 2007.

The 26-year-old worked as security guard for G4S in Florida, where the FTSE-listed company provided contractors to securing federal buildings.

G4S said that Mateen would have carried a gun as part of his duties and had undergone background checks by the company in both 2007 and 2013 and by a US law enforcement agency.

The company confirmed that the US-born security guard worked at a residential community in South Florida and was off-duty at the time of the incident.

Reports on Monday said Mateen made mention of the so-called Islamic State in a call with emergency services during the attack on Sunday, though details about his links with the terror organisation have not yet become clear.

“That G4S has employed a man who’s reportedly been investigated by the FBI three times, once for alleging terrorist ties to co-workers doesn’t reflect well at all on the company’s business practices,” said Jasper Lawler, market analyst at CMC Markets.

“If the name of G4S starts getting dragged through the mud, US contracts may become harder to come by. That would be a serious hit to the company’s bottom line.”

The company's statement conveyed that it was "deeply shocked by the tragic events" and that "the thoughts of everyone at G4S are with the victims and their families".

G4S said Mateen was subject to detailed company screening when he was recruited in 2007 and re-screened in 2013 with no adverse findings. He was also subject to checks by a US law enforcement agency with no findings reported to G4S.