There are currently 85,076 inmates in England and Wales
The prison population in England and Wales has reached a record high, exceeding 85,000 for the first time.
There are currently 85,076 inmates, compared with 84,897 in March. If the rise continues at that rate, the total will surpass 85,700 by the end of June.
But there are still 2,000 places left in the "useable" capacity of 87,196.
The increase is likely to be partly due to the phasing out of the early release scheme, which allowed some inmates to be let out 18 days early until 9 April.
Prison chiefs began using the scheme in June 2007 amid rising prisoner numbers.
Between then and February 2010, about 80,000 inmates - serving up to four years for a range of less serious offences - have been freed early.
'Out of control'
In February, Justice Secretary Jack Straw announced the scheme would start being phased out from 12 March 2010.
Mr Straw told MPs that there were now enough spare places in the prison system to mean it was no longer necessary to consider any prisoners for the scheme.
At the time, he said the prison service was on track to have 96,000 spaces available by 2014 because of the current building programme.
This ceaseless growth in prison numbers is untenable and any new administration will have to bite the bullet and find a strategic way to reduce the prison population
Howard League for Penal Reform
BBC home affairs correspondent Danny Shaw said Friday's figures, confirmed by the Ministry of Justice, showed the prison population in England and Wales had gone up by about 24,000 since Labour came to office in 1997.
Juliet Lyon, director of the Prison Reform Trust, said the cost of sustaining the "out of control" population had "gone beyond affordable bounds".
"Each new prison place costs £170,000 to build and maintain, and the cost per prisoner per year is £41,000.
"Instead of planning how to cope with ever rising prison numbers, politicians should be working hard to reduce any unnecessary use of imprisonment.
"This would mean less breach and remand, fewer mandatory terms, a review of indeterminate sentences and proportionate sentencing guidelines," she said.
Frances Crook, of the Howard League for Penal Reform, called for the leaders of all political parties to set out their policies to tackle numbers behind bars ahead of the general election.
She said: "This ceaseless growth in prison numbers is untenable and any new administration will have to bite the bullet and find a strategic way to reduce the prison population."