The cost of security for the 2012 London Olympics and Paralympics has almost doubled to £553 million.
The massive operation to provide security at more than 100 venues across the UK has swollen by £271 million after a decision to recruit and train almost 14,000 more personnel, said the Department for Culture, Media and Sport.
Meanwhile, the price tag for the opening and closing ceremonies has risen by £41 million, also doubling previous forecasts.
DCMS insisted that the decision to increase the number of security staff from 10,000 to 23,700 was not in response to "any specific security threat".
Olympics minister Hugh Robertson said that the additional spending would not put the Games over their £9.3 billion budget, thanks to savings of more than £910 million achieved by the Olympic Delivery Authority since 2007.
With 235 days to go to the July 27 opening ceremony, the Olympic programme remains "on time and within budget", with construction of stadiums and infrastructure now 92% complete, said Mr Robertson.
The Government and the London 2012 organising committee (LOCOG) have now undertaken detailed analysis of the number of security staff required, he said.
"As a result, to ensure a safe and secure Games, they have revised the numbers of trained staff required," he said. "We are therefore investing additional funds in providing nearly 24,000 venue security personnel plus specialist security equipment."
Mr Robertson said the decision to provide LOCOG with extra public money for the opening and closing ceremonies was taken to take advantage of "a great national moment" which could generate advertising revenue of £2-£5 billion.
LOCOG's budget for the ceremonies has never been officially confirmed but was previously understood to have been £40 million.
Mr Robertson said: "London's opening and closing ceremonies are a once-in-a-generation opportunity to showcase the very best of our country to four billion people around the world.
"To get the ceremonies absolutely right and boost the Games business and tourism legacy, we are putting additional investment into our ceremonies."
Unveiling the latest quarterly report on the cost of the Games, Mr Robertson revealed that the ODA had achieved a further £42 million in savings between July and September.
The overall funding package remains at £9.298 billion, with more than £500 million of spare cash available for contingencies.
But additional cost pressures include £271 million for security, £41 million for opening and closing ceremonies, £2.8 million for command, co-ordination and communications testing and £25 million for campaigns to boost the economic benefits of the Games, including tourism.
The anticipated final cost of construction, infrastructure and transport for the Games now stands at £6.856 billion - down £394 million since July. Including funding for transformation work, this rises to £7.189 billion - down £61 million.
Contracts for the sale of the Olympic Village and adjacent development plots to the Delancey and Qatari Diar joint venture have been exchanged, generating £557 million for the ODA.
Mr Robertson said: "The Olympic programme remains on time and within budget.
"Consistent careful management of the finances has enabled us to fund additional costs such as venue security from within the public sector funding package as well as to invest in projects that will help drive economic growth from the Games.
"This has been a big year for the programme, with the construction project reaching its peak and the staging of the Games coming to the fore.
"As we get ready to enter the Olympic year and the pressure mounts, I am confident that we remain on track to deliver a memorable Games that will enthral the nation and the world."
ODA chief executive Dennis Hone said: "We're in the home straight, with the finish line in sight - and still going flat out to keep down the bill for building the Games.
"We can now report that more than £900 million has been saved and our anticipated final cost cut again. But there is still work to do, with smaller venues to be completed and the Olympic Village set to be finished around the end of the year.
"So we are anything but complacent and totally committed to securing value for money for the public from this huge project."