Security caught off guard at the Tower as thief makes off with the keys

There was embarrassment at the Tower of London - home of the British crown jewels - after it emerged that guards released an intruder they found on the grounds, without realising that he had stolen a set of its keys.

The locks have since been changed. It is certainly not how they used to treat criminals at the Tower of London, where grisly executions have made the 940-year old fortress a symbol of ruthless British justice.

It emerged on Monday that instead of a night in the dungeon or a trip through Traitors' Gate, private security guards at the royal palace mistakenly released the night intruder, who they found prowling in the grounds.

They missed the fact that he had stolen a set of keys to drawbridges and other locks.

In the early hours of November 6, as the embers of Bonfire Night - when Britain's celebrate Catholic rebel Guy Fawkes' 1605 attempt to blow up the House of Parliament - died away, the intruder scaled a fence and dropped down into a pedestrianised area.

The man's presence was detected by the guards in the security control centre and the night guard was despatched to apprehend the trespasser and was on the scene within three minutes. By then, though, the intruder had managed to swipe a set of keys from an unguarded sentry box just outside the main gates.

The Tower authorities sheepishly admitted on Monday that instead of getting to the bottom of what he was doing there and what might have been in his pockets, the guard led him from the premises and let him go.

The thief remains on the loose, the keys are nowhere to be found and the red-faced managers of the royal palace are facing a bill of several thousand pounds for changing the locks.

The Tower's famous red- and black-clad guard yeoman warders, known as Beefeaters, were not to blame. The perimeter area where the incident happened was patrolled by a private security firm, ICTS, which won a contract for the task in 2010.

Kelly McCartney, commercial director of ICTS, said the firm was "not making any comment" on the break-in.

A spokeswoman for Historic Royal Palaces, which is the custodian of the Tower of London, said: "During this incident, keys for a restaurant and conference rooms were taken together with a key to an internal lock to the Tower drawbridges that is not accessible from the outside.

"It would not have been possible to gain access to the Tower with any of these keys and at no point was the security of the Tower at risk. All affected locks were immediately 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. A staff disciplinary procedure is under way."

The Metropolitan Police said: "An allegation of theft has been made to police, which is being investigated." In 1671, Irish colonel Thomas Blood pretended to be a parson to trick his way into the room where the crown, orb and sceptre were kept and knocked the guard unconscious before trying to make off with the treasures, crushing the crown into a bag and stuffing the orb down his breeches.

"He was arrested at the scene after trying to shoot another guard, but was pardoned by King Charles following a trial.

One of the main attractions for visitors is the nightly "ceremony of the keys", the locking up of the Tower of London which has taken place every night for at least 700 years.

The stolen keys were a different bunch to those used in the ceremony, but the irony is not lost. The Tower's publicity material for the ritual states: "The importance of securing this fortress for the night is still very relevant because, although the monarch no longer resides at this royal palace, the crown jewels and many other valuables still do!"