HMP Dartmoor, which is owned by the Duchy of Cornwall, is leased to the Ministry of Justice for £1.5 million annually - and will be for the next 24 years.
The EC said there had been 'shortcomings' in Britain's implementation of treaties, with allegations centering on Britain's failure to comply with EU laws on the free movement of people.
Ministry of Justice data shows an all-time high of 87 people were let out from behind bars too early last year - and more than 300 over the past five years.