A thief reportedly managed to escape with keys from the leading tourist attraction on Guy Fawkes Night.
The unidentified raider, who has not been caught, is said to have managed to scale the Front Gate as well as an inner entrance.
They then managed to grab a set of keys from a metal box that had been mistakenly left unlocked at a sentry post, the Sun reported.
Tower officials insisted that the Crown jewels were never at security risk but admitted that staff could lose their jobs over the fiasco.
Despite being spotted by guards, he escaped because the security team have strict instructions not to leave their posts.
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The newspaper reported that the radio pleas for help requesting back-up were ignored during the incident on Bonfire Night.
It was claimed that the "heavy" bunch of keys included those to unlock drawbridges, conference rooms and the restaurant.
As a result of the embarrassing security blunder the locks had to be changed, at an estimated cost of thousands of pounds.
Scotland Yard launched a huge manhunt but failed to find the suspect who targeted the tower, which is guarded by Yeoman Warders, or Beefeaters. They are supported by a private security firm.
A source told the newspaper: "Security was a total shambles. The burglar climbed over the Front Gate then got over another gate and found a metal box with the keys inside.
"The box was supposed to be locked at all times and it was open.
"It went from bad to worse. Guards saw him but could not leave their positions to chase him. They tried to radio a night watchman but got no response."
Last night a spokesman for the Historic Royal Palaces, which manages the Tower, admitted that security procedures "were not carried out to the expected standard".
He added: "A staff disciplinary process is underway." They did not comment further.