The use of electronic tags on offenders has increased, as the number of prisoners has dropped to its lowest level in seven years, figures show.
The number of prisoners released early on electronic tags rose by more than 1,000 in seven months to 3,028.
The Ministry of Justice said there are about 83,000 prisoners in England and Wales, down 2,000 in four months.
BBC home affairs correspondent Danny Shaw said the rise in tag use had contributed to the sudden drop.
He said while jail staff would welcome the spare capacity in prisons, some governors have privately expressed concern that the expansion in the use of the tagging scheme has removed some of their discretion over the release of prisoners.
It may also fuel fears that some offenders are being let out when they are not ready for release and may go on to commit further crimes, our correspondent added.
The BBC understands this is the first time the number of offenders on electronic tags has surpassed the 3,000 mark since the tagging scheme – known as HDC (home detention curfew) – began in 1999.
The last time the prisoner population was this low was in January 2011.
HDC – introduced by Labour – sees prisoners freed between two weeks and four-and-a-half months before their automatic release date.
They are made to wear a tag and abide by a curfew.
In February, it emerged that ministers had simplified the rules around granting HDC to ensure it became a “normal part of release”, because only a fifth of eligible offenders were being let out.
The scheme is available for those sentenced to at least three months’ imprisonment.
HDC is not available for sex offenders, people convicted of terrorism offences and certain violent criminals.