The head of private security giant G4S is to face a grilling from MPs on Tuesday after the company failed to provide enough security guards for London\'s Olympic Games. An extra 3,500 troops have been drafted in to plug the gap after the contractor admitted just two weeks before the Games that it could not provide the 10,000 guards it had promised. It also emerged Monday that at least eight police forces have had to deploy extra officers at Olympic venues across Britain -- reportedly after employees of G4S, one of the world\'s biggest defence firms, failed to turn up to work. The company\'s chief executive Nick Buckles was to appear before parliament\'s Home Affairs Select Committee amid mounting pressure for him to quit his £830,000-a-year job. G4S shares slid 1.85 percent to 249.90 pence in early morning trade on Tuesday, after suffering heavy falls on Monday. Home Secretary Theresa May was unable to put a figure on the G4S staffing shortage, insisting that the \"precise balance\" of military and private personnel would become clear in the coming days. In a heated parliamentary debate on Monday, May said it was \"untrue\" that ministers knew last year that there would be a shortfall. \"G4S repeatedly assured us that they would overshoot their targets,\" she said. The extra troops pledged by the government last week brings to 17,000 the total number of military personnel lined up for Games security. G4S, which is set to lose up to £50 million for failing to meet the obligations of its £284 million Olympics contract, insisted that the extra police should only be needed for a few days. \"This situation is being rectified over the coming days, which should lead to the withdrawal of police officers from those roles assigned to private security,\" a G4S spokesman said. Culture minister Jeremy Hunt, who has repeatedly defended G4S, told BBC television on Monday that now is not the time for a \"witch-hunt\" and insisted the Games would be \"safe and secure\". The government came under further pressure Tuesday as a report said that the Border Agency -- which has come under fire in recent weeks over hours-long immigration queues at London\'s Heathrow airport -- has cut too many employees. Over 1,000 staff more than initially planned have lost their jobs in the last year and the Border Agency has now been forced to hire extra people to deal with the workload, the National Audit Office said. It comes after the first athletes and Games visitors began arriving at Heathrow on Monday and the Olympic Village opened in Stratford, east London.