A set of five keys went missing on Wednesday

Work was being carried out over the weekend to secure the perimeter of Channings Wood Prison, near Newton Abbot, as well as all the external locks.

And on Monday work will start to replace the locks on 594 cells at the category C prison.

The final cost is expected to be in the region of �250,000.

Full inquiry

The alarm was raised when a set of five keys went missing on Wednesday.

More than 600 prisoners were immediately locked in their cells while a thorough search was made.

Prison bosses say there was no risk to security.

Governor Nicholas Evans said it was not thought the keys had come into the possession of any prisoners.

About 500 sets of keys also need to be replaced.

It is the second embarrassing security incident for South West prisons in recent weeks.

Only three weeks ago three prisoners escaped from Dartmoor Prison.

Two have been recaptured, but one is still on the run.