The prison population could edge towards the 90,000 mark in the next five years, according to official projections that sparked warnings the system is "buckling under the weight".
As of last Friday there were 86,388 people behind bars in England and Wales which was up by more than 1,500 compared with 12 months earlier.
New figures compiled by the Ministry of Justice show the tally is projected to remain stable until June 2019, before increasing by 1,600 above the current total to 88,000 in March 2022.
The headline projections are based on a 50% likelihood of the population reaching or passing a given level.
There is a 20% likelihood it will reach or exceed 89,700, and only a 5% likelihood it will reach or exceed 91,800, in June 2021, the report added.
The number of inmates has come under sharp focus in the last year as jails were hit by surging levels of violence, self-harm and drug use.
The population projections were released weeks after Justice Secretary David Lidington said he wanted to see prison numbers come down.
"We need better custody that cuts re-offending and crime," he wrote in the Evening Standard.
"And we need to ensure judges, magistrates and the public have full confidence in the other penalties available."
Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said the number of people behind bars has more than doubled since Margaret Thatcher was in power.
He said: "Our failed prison system is buckling under the weight.
"The Government must reduce demand on its jails if it wants to regain control of them.
"Bold but sensible action to reduce the prison population would better protect the public and prevent more people being swept into deeper currents of crime, violence and despair."
Peter Dawson, director of the Prison Reform Trust, added: "These projections scupper the government's plans to build its way out of the prison crisis.
"It may be politically uncomfortable, but there is no option now but to have the debate about who we send to prison and for how long."
A Ministry of Justice spokeswoman said: "We will always have enough prison places for offenders committed to custody by the courts.
"Prison numbers can fluctuate which is why we have a robust set of plans in place.
"This includes modernising the estate and building new accommodation.
"We are also transforming our prison estate and investing £1.3 billion to deliver 10,000 new places."