London is expected to see its largest ever security operation in 2012
A lack of security staff for London's Olympics means veteran police officers must be brought out of retirement, a senior police figure has said.
More than 1,500 officers will also have to be brought to London from other parts of the UK to guard the Games.
Cuts to police dogs and horses will now have to be reversed to provide the level of protection required, Assistant Met Commissioner Tarique Ghaffur said.
Organisers and ministers will receive a full report on security on Thursday.
Officials and senior police officers have been meeting to discuss arrangements for the Olympics, when a huge security operation will be required.
This will include a 5,000-strong police presence, assembled mostly from London-based officers but also, it has now emerged, from other UK forces.
Mr Ghaffur, who is overseeing security at London's Games for the Metropolitan Police, told the meeting there could never be enough personnel for security.
Met Assistant Commissioner Tarique Ghaffur oversees Olympic security
He said his force was asking all of London boroughs to share their resources for 2012.
The meeting also heard from David Evans, the project director of the British Security Industry Association, that the bid submitted for the Games had underestimated the personnel required from that sector.
The bid estimated that 6,500 would be available, but in fact only 1,000 private security guards are available at present, suggesting far more training will be required in the next four years.
Mr Evans also highlighted security problems in previous Games, such as an explosion at the Olympic Park in Atlanta.
At this venue, the stewards were volunteers who ran away after the blast and did not provide policing needed, he said.
There were also security issues at the Winter Games in Turin in 2006, during which the police had to intervene, Mr Evans added.
About £830m of the total £9.3bn Olympic budget has been allocated to security.
The aquatics centre is part of the big Olympics complex at Stratford
The Home Office is working to ensure that an estimate of the full security costs is ready by October.
Meanwhile the Olympic Delivery Authority has admitted it may have to change some of its plans to build a stingray-shaped roof on the aquatics centre to be used at the Games.
The venue's budget has already quadrupled to £300m.
The main contractor involved has been told it would be difficult to obtain the large quantities of wood needed for the roof, and that this wood remained untested in damp conditions, BBC London has learned.
"We're prepared... to work with the architect and the contractor to ensure that we deliver a facility that works perfectly for the Games and works perfectly for the long-term legacy of the building and the local community," said authority executive John Nicholson.