Britain's army will have to provide extra troops to police Olympic venues as a private security firm cannot supply enough trained staff, while a watchdog warned on Thursday that border staff are under-trained.
Responding to media reports that the military had been asked to provide up to 3,500 extra personnel, the country's interior ministry acknowledged Wednesday that it had intervened "by revising the level of military support" for the summer games which open late this month.
Defence Secretary Philip Hammond would announce details of the new plans to parliament through a written statement on Thursday, the Home Office said.
"We retain the ability to be flexible in our response," the statement said.
"We have agreed to offer help to G4S by revising the level of military support. The focus of the government and everyone involved is on delivering a safe and secure Games."
The government insisted it was confident of sticking to the £553 million ($877 million, 662 million euros) budget for venue security.
A security force of more than 40,000 people, backed by a huge intelligence operation, is to guard the Games, which start on July 27.
The Ministry of Defence was due to provide 13,500 troops, but could now be asked to supply 16,500. They will work alongside police, private security guards and unpaid volunteers.
A G4S spokeswoman admitted they had "encountered some issues in relation to workforce supply and scheduling."
Shadow Olympics minister Tessa Jowell said it was "imperative that action is taken to ensure that the full and necessary quota of security personnel are in place".
"But this is clearly a serious problem, and we have to understand how this problem arose."
Meanwhile, border staff drafted in to cope with the influx of visitors to Britain for the Games may lack proper training and immigration experience, an independent watchdog warned.
John Vine, chief inspector of borders and immigration, said that extra staff brought in to ease congestion at London's Heathrow Airport, the main gateway to the Games, were taking longer to process visitors despite asking fewer questions.
According to Vine's report, some staff "remained concerned about the potential risks of employing staff on the immigration control who had received only basic training and who had no immigration background/experience".
"Staff also expressed concerns about similar problems recurring after the Olympics, if resources were not sufficient to meet the increasing passenger flows coming through Heathrow," he added.
UK Border Force, the interior ministry agency responsible for protection of the country's borders, is also re-hiring former employees as it prepares to deal with an expected 100,000-a-day spike in arrivals, Vine revealed.
Around 500 extra staff have been brought in to ease immigration queues, but an inspection in April found that the new officers "appeared less confident when processing passengers".
In another pre-Games scare, Britain's Highways Agency on Wednesday said it expected the main road between Heathrow Airport and London to reopen in time for the Olympic Games despite ongoing repairs to a cracked viaduct.
The road will be the main artery for transporting athletes and officials from Britain's busiest airport to the Games, which begin on July 27.
The M4 motorway was closed in both directions in west London and will remain so for "the next few days," according to a statement issued by the government agency.
"The final complex work has reached a critical stage and, due to the complexity of the operation, the road will remain closed," it said.
"Conditional on the repair work being successful, it is still expected that the work will be completed and the road reopened within the next few days.
"The Highways Agency and its contractors are working around the clock and will reopen the motorway as soon as it is safe to do so," it added.