Predictions for the prison population suggest government building plans may not provide enough cells for inmates.
The Ministry of Justice figures suggest there will only be enough cells in the most optimistic of circumstances.
More than 80,000 people are in prison and the government has pledged 9,500 extra spaces to ease overcrowding.
Ministers have also published separate figures showing 3,832 prisoners have been let out of jail since June under a special early release scheme.
The current prison population is hovering at just below a record 81,000, with police cells being used for some inmates as an emergency measure.
‘Too few cells’ under prison plan