Shares in G4S plunged on Monday after the security group admitted that its failure to provide enough guards at the London Olympics would cost it up to £50 million.
In early morning deals, the company's share price dived by about 10.0 percent in value. It later stood at 254.7 pence, down 8.61 percent from Friday's closing level.
London's FTSE 100 index of top companies, on which G4S is listed, was 0.27 percent lower at 5,650.72 points in late morning deals.
G4S chief executive Nick Buckles was forced to apologise on Saturday for recruitment blunders that have forced the British government to deploy extra troops to guard the London Olympics at the last minute.
Buckles said that he was "very sorry" that 3,500 troops have had to be drafted in just two weeks before the Games, after the company admitted it could not provide the total 10,000 guards it said it would.
Speaking publicly for the first time since details of the fiasco emerged last Wednesday, Buckles confirmed that G4S faces losses of between £35 million and £50 million in the current financial year as a result of the blunder.
"We accept that we underestimated the task of supplying staff for the Olympics. We deeply regret that," Buckles told BBC radio on Saturday.
He admitted that he had only realised "eight or nine days ago" that there would be a shortfall.
"It's only when you get closer and closer to the Games that you realise that the number isn't as high as you expect," he said.
In reaction to the stream of negative news, Seymour Pierce stockbrokers removed its "buy" rating on the stock and cut its profit forecasts for the current financial year by £60 million.
"It appears certain that Mr Buckles will fall on the sword along with other senior UK management," said Seymour Pierce analyst Kevin Lapwood.
"This could lead to a period of instability at the company which appointed a new chairman just over a month ago.
"Whoever is in charge will have a lot of work to do to repair the company's reputation, especially at home in the UK and with the UK Government, its single largest customer. This will be crucial for the company's future."