Hundreds of education staff at more than 70 prisons and young offender institutions across England are on strike in a row over new contracts.
The University and College Union (UCU) is unhappy with the terms and conditions set out by The Manchester College (TMC), which runs offender education courses.
It fears increased working hours and holiday and sickness entitlement cuts.
TMC said the changes would ensure the "long-term viability" of the service.
The UCU claimed some staff could lose up to £7,000 a year under new pay scales.
Union president Alan Whittaker said: "Our members don't want to take strike action, but they have been left with no choice.
"It is deeply concerning that Britain's largest prison education provider is trying to force through new contracts that will seriously jeopardise prison education in this country."
Of the 3,500 staff employed by TMC's offender learning department, 894 are members of the UCU.
TMC said 355 of the 457 members who voted had opted to strike.
A TMC spokesman said the college was "disappointed" that UCU members had decided to strike during negotiations.
A spokesman said: "Our aim is always to use public money wisely and efficiently, to reduce the number of job losses and treat our employees fairly, and to maintain excellent quality and quantity of offender learning within our establishments."
The UCU held a rally at the Manchester Conference Centre Institute at 1200 BST where Mr Whittaker and prison teachers made speeches.
The Prison Service said it anticipated some disruption to learning, but minimal disruption to the running of the affected institutions.
- The areas affected by the strikes are: Berkshire, Buckinghamshire, Cheshire, Cumbria, Greater Manchester, Isle of Wight, Kent, Lancashire, Merseyside, Milton Keynes, Oxfordshire, Shropshire, Staffordshire, Surrey, Sussex, Warwickshire, West Midlands, Winchester, Worcestershire, Yorkshire and the north east of England.