HMP Haverigg had to pay a reported £80,000 to change the locks after the internal keys went missing at the beginning of the month.
However - as is usually the case - the keys turned up just a few days later, meaning their big bill was in vain.
A spokesman said that only internal locks had been replaced and no official cost of the task was available. Despite this, an insider has estimated the cost to be up to £80,000.
Although the missing keys could spark worries over security, the Ministry of Justice have insisted the "human error" posed no security risk or public threat and that 644 inmates were said to be in their cells at the time.
The prison service told the Daily Mail that no members of staff have been disciplined over this incident. They said: "Incidents like this are extremely rare. We always investigate them thoroughly and update our security measures accordingly."
By Anna Patterson
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