The move at the jail - once called Strangeways - has been prompted by an official video which shows close-up footage of keys being used in locks.

Strangeways prisoners take to the roof in 1990

The film was made to be viewed by new inmates and has been distributed to local community groups.

But prison managers feared if it fell into the wrong hands replicas of keys could be made from freezed frames.

It is understood concern was caused by one particular sequence in the four-year-old film showing a chaplain doing his rounds and featuring a close-up of his bunch of keys.

Although the film was made to be viewed by new prisoners, it is not known if inmates ever saw it.

The Prison Service said there was no evidence it caused a security breach.

Prison in the news

The massive lock-changing operation was ordered as a precaution.

A spokesman said: "The mistake has only recently come to light. The prison has not experienced any related security problems since."

He said it was not known how much the operation would cost.

HMP Manchester was known as Strangeways when it was the scene of a 25-day rooftop siege in 1990, although the latest Chief Inspector's report said it had improved a great deal since then.

The jail was in the news last month when two inmates died there.

Their deaths are being investigated.