The Orlando shooting carried out by an employee of security firm G4S has wiped £200 million off the value of the company.
When trading opened in London on Monday, shares in G4S lost 6.6 percent to 175 pence to hit their lowest level since 2009.
Omar Mateen, 29, an armed security officer for the firm had undergone company screening as recently as 2013 with "no findings."
The incident could cause yet more damage to G4S after a long line of issues which included not providing enough guards for the London Olympics.
They were also involved in a prison tagging fiasco and took a £65 million charge on loss-making British government contracts.
Employees at a gun shop refused to sell Omar Mateen bulk ammunition because he was acting suspiciously.
The Orlando mass shooting - the worst in modern US history - has reignited the gun control debate in America.
Omar Mateen is said to have called Orlando's News 13 about 45 minutes after the shooting at the Pulse nightclub began.