The Tower of London has been forced to change its locks after a burglar stole keys to the fortress on Guy Fawkes night, UK media have reported.
The Sun said the thief scaled two gates before ransacking a metal box - which was supposed to have been locked - for the keys.
The newspaper said the burglar was spotted by guards who radioed for help - unable to leave their posts - but were ignored and the man escaped.
It said police unsuccessfully hunted for the man. The BBC reported that he had been found, though no arrests had been made.
Tower officials said the Crown Jewels had not been at risk, the Sun reported. The keys were said to include ones to unlock drawbridges and conference rooms.
A spokesman for the Historic Royal Palaces told the paper: "A staff disciplinary procedure is under way."
A spokesman later told the Daily Mail's website it would not have been possible to gain access to the Tower with the keys and its security had not been at risk.
He told the MailOnline that all affected locks had been changed.
"We have carried out an internal investigation and have concluded that our well-established security systems and procedures are robust. However on this occasion, these procedures were not carried out to the expected standard," the spokesman said.