A senior police officer left security plans for the London 2012 Olympics on a commuter train, the Sun has reported.
'Restricted' files containing details of pre-Olympics rehearsals, an explanation of emergency "lock-down" procedures and plans to avoid traffic congestion were revealed in the dossier, which was found on a train in Dartford, Kent.
The worrying slip by the officer included minutes of meetings where anti-terrorism plans were discussed, as well as names and phone numbers of senior police officers involved in security for the London 2012 Olympics.
The files were handed to The Sun by a commuter, which the paper then returned to the police. The files were purported to reveal police frustrations concerning Olympic communication. One police officer is minuted as saying "radios never work properly", while another damns the equipment, calling it "completely unreliable."
Scotland Yard played down the oversight, saying security for the London Olympics had not been compromised. A Metropolitan Police spokesman insisted the documents were not thought to be "operationally sensitive".
The Met spokesman said: "On Thursday, January 5 a Metropolitan Police Service officer lost his bag containing a number of documents.
"The officer reported the loss to a senior officer. Directorate of Professional Standards have been informed as is routine.
"We do not believe that the bag contained operationally-sensitive documents.
"The documents are now back in police possession."
Security plans for the 2012 Olympics are coming under increasing scrutiny as the £9.3 bn budget apportioned for Games continues to increase in part due to spiralling security costs.
When London bid for the 2012 Olympics in 2005 the spend was estimated £2.4 bn.
In December, the National Audit Office warned that Olympic costs could even surpass the now tripled budget because of rising security spend.
Last month the government said an extra £271m was needed for security guards. This puts the total security spend up to £553m.
This means that the contingency amount set aside for 2012 Olympics is now "a wafer thin" £36m, according to chairman of the Commons Public Accounts Committee, Margaret Hodge.
Despite the amount spent, there are concerns that the security so far is not enough to protect the site.
A cabinet meeting held in the handball arena at the Olympic stadium in Stratford, East London on 8 January, was overshadowed by news that the previous year, a fake bomb had managed to make it through security onto the site.
A trial security run failed last week after sniffer dogs failed to identify 20 kilos of explosives.