Overall, there was a 14% increase last year.
There was a 14% increase in the amount of serious complaints made by prisoners last year.
The highest number of complaints, since the beginning of 2018, were made in the Midlands Prison.
According to details released under the Freedom of Information Act, there were 70 in 2017 and 80 in 2018.
But since the beginning of 2017, only 16 have been upheld or partly upheld.
Gabriel Keaveny, assistant general-secretary of the Prison Officers Association, says vexatious complaints should be punished: