It is believed the 25-year-old had extensive head injuries from a beating about 6.30pm on Saturday.

He was taken to the Canberra Hospital where he was treated and transferred back to the prison the following day.

Police said yesterday charges would to be laid over the incident.

The Canberra Times understands two prisoners are being investigated over the assault.

It's understood one of those on remand is awaiting trial on a range of offences including possession of a firearm, driving offences, bail breaches and theft charges. The other is awaiting trial on 10 charges related to burglary, theft and riding in a stolen vehicle.

In a statement, Inspector Howard Jones, from ACT Corrective Services, said the man had received care at the prison before paramedics transferred him to hospital.

''The matter has been referred to the AFP and is currently under investigation as is normal procedure,'' Mr Jones said.

ACT Corrective Services did not disclose yesterday the extent of the prisoner's injuries or how long it took prison officers to respond.

The corrective services spokeswoman said the prisoner would be back in his cell, with the prison's medical facility not staffed around-the-clock.

She was unable to confirm whether the two men believed responsible for the attack had been placed in segregation, but said the action would be in line with the normal course of events.

A police statement issued yesterday said two prisoners, aged 26 and 29, would be charged on summons with assault occasioning actual bodily harm.