Around one in ten inmates at Exeter prison was caught with a mobile phone last year.
Figures released today show that officers confiscated 48 mobiles in 2017 from a prison population of nearly 500 men at Exeter's New North Road jail.
Mobiles are used by some prisoners to continue criminal activities from the inside, including delivering drugs to inmates.
Three years ago, just seven mobiles were found at Exeter.
Dartmoor had proportionately less of a problem last year; where mobiles were found on one in around 20 prisoners.
The figure for Channings Wood prison near Newton Abbot saw one in eight inmates having access to devices.
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