The University and College Union says the education of inmates and their chances of keeping away from crime on release could be disrupted by plans announced by John Reid, the Home Secretary.
Under Operation Safeguard, the Home Office plans to transfer some prisoners to police cells or detention centres to reduce the pressure on the system.
Christiane Ohsan, the UCU's national official for prison education, said:
"Prisoners transferred to police cells will not be receiving any education or training.
"And the frequent transfer of prisoners between institutions can set back to square one their attempts to pursue education.
"Prisoners who don't receive education are three times more likely to be convicted again than those that do, so these emergency measures are going to raise reoffending rates."
The emergency measures were announced by Mr Reid after the prison population approached capacity. They include making 500 police cells available.
It is thought the numbers have since dipped, meaning police cells would be unlikely to be used except in rural areas.
The prison population of England and Wales this month reached a record 79,843, theoretically leaving space available for only 125 more prisoners.
Most prison education contracts are held by FE colleges. The UCU's concerns follow a report by the Learning and Skills Network which claims women are getting a raw deal from prison education.
An LSN study suggests there needs to be more tuition aimed specifically at women, covering family responsibilities and dealing with their emotional needs.
They can be greater because they are more likely to be in prisons far from home.
The ability of prisoners to make their views heard in the outside world has diminished in recent weeks, although the UCU does represent many of their tutors.
The Prison Education Forum, the charity which had advised ministers, has gone into receivership after the discovery of alleged financial irregularities now being investigated by police.