The number of prison officers at Lincoln prison was cut by 37 per cent in three years.
Figures from the Howard League for Penal Reform show that there were 207 officers working there in September 2010, but three years later that number had dropped to 130.
At North Sea Camp, near Boston, numbers reduced 29 per cent from 56 to 40. The charity warns that it could mean riots and attacks in prisons become more common as a result.
Prison officers from across the region who had walked out earlier have now ended all protest meetings.
They were highlighting their opposition to the Government's plans to link the normal pension age with the state retirement age.