The prison population of England and Wales has hit a new record high of 86,608 people, with a rise of nearly 700 this week as the courts took the exceptional step of remanding into custody almost two-thirds of those charged with riot-related offences.

Prison governors said that the system now faced "an unprecedented situation" because of the riots, and emergency contingency measures had been agreed with prison service chiefs in case the rise in inmate numbers continued unabated.

The Prison Governors Association said the medium- to long-term measures included opening sufficient new and refurbished jail accommodation to avoid the normal emergency measure of using police cells.

The governors said they were confident the situation could be managed safely.

The record prison numbers are putting the jails and young offender institutions under increasing pressure; there are only 1,485 spare places in the system before prison governors have to put out the "jail full" signs.

Prison service chiefs are expected to outline the new contingency measures on Friday, including increased overcrowding by doubling and even trebling inmates in cells designed for single occupation.

Prison governors had already warned that the riots have put further strains on an already-stretched prison system, with inmates being moved out of London and Manchester to create space for rioters being sent to jail or remanded in custody awaiting trial.

The Ministry of Justice said that its latest figures, up to noon on Wednesday, showed that 1,297 people had appeared before magistrates charged with riot-related offences. A total of 772, or 65%, had been remanded in custody, compared with the "normal" remand rate for serious offences being 10%.

"This is causing massive problems for prisons," said Harry Fletcher, of Napo, the probation officers' union. There are so many of them coming through the system, it is causing considerable problems. When people are being held so far from home it causes real difficulties for their families." He said that Nottingham jail alone had been sent a group of 30 prisoners from London this week.

The total prison population on Friday last week stood at 85,931, which included 607 immigration detainees. As space runs out so the potential for work, education or rehabilitation will be "zero", claimed Fletcher.

The justice secretary, Kenneth Clarke, will be hoping that the developing pressures on the prison system are purely temporary, otherwise they have the capacity to derail his plans to stabilise the jail population and bring in his "rehabilitation revolution".

The normal pressure valve for the prison system when it comes close to capacity is to put into effect Operation Safeguard, which involves emergency use of police cells to house prisoners. But that option is now closed off as forces stay prepared for any further disturbances.

In the medium-term the prison service might be able to add portable accommodation within the perimeters of existing jails, and no doubt in the longer-term the prospect of finding a new prison ship could be raised.

The prison service has already announced plans to close two small jails, Latchmere House, in London, and Brockhill prison, at Redditch, Worcestershire, next month. One option could be to postpone these closures if the pressure on jails continues to rise at the current rate.

A Ministry of Justice spokeswoman said that there were enough places for those being sent to prison, including in young offender institutions, following the riots: "There is substantial capacity in the prison system. We will provide prison places for those committed to custody by the courts. We are developing contingencies should exceptional pressure be placed on the prison estate."