Shares in security firm G4S slumped to their lowest since 2009 after it emerged that the gunman who killed 50 clubbers in a Florida nightclub this weekend was an employee.

Omar Mateen, an American citizen who pledged allegiance to Islamic State before embarking on the massacre at the Pulse nightclub in Orlando, had worked for the global giant since 2007, the company confirmed.

Mateen was twice cleared by G4S’s checks and the company admitted it was aware of his being questioned by the FBI over potential terror links in 2013.

The revelation deals another blow to a company whose reputation has been buffeted in recent years by scandals over tagging dead prisoners and failures to provide enough security guards at the Olympics.

G4S, down 11.2p to 176.2p, sank as much as 8% at one stage as the City worried about the potential repercussions of the US’s worst gun massacre on the business.

Russ Mould, investment director at stockbroker and investment manager AJ Bell said the fall in the share price seemed a “fairly dramatic move”, potentially exacerbated by market jitters over a potential Brexit.

But he added: “I’m sure there will be questions asked about how it selects its staff and how it screens them.” “The shooting in Orlando could open up a can of worms for G4S,” added CMC Markets analyst Jasper Lawler.

G4S’s regional North American boss John Kenning, who said the business was “deeply shocked by this tragic event”, confirmed Mateen had been employed by the firm since September 2007, working at a gated retirement community in south Florida.

He added that the gunman had been through company screening and background checks when he was recruited, revealing no concerns. The screening was repeated in 2013 with no findings.