Only prisoners serving less than four years are eligible for tagging

The National Association of Probation Officers (Napo) said governors were being told to free more prisoners - even those considered likely to reoffend.

An extra 1,000 prisoners would be freed every month - raising the monthly average to about 2,600, says Napo.

It said it was "a panic measure" as prison numbers have risen to 75,000.

The union's magazine, Napo News, said the only prisoners eligible for release under the home detention curfew scheme who would remain behind bars were those posing "a risk of harm to the public".

The number sent back to prison for breaching their curfew could jump from 288 in October to 500 or 600 by the summer.

There has certainly not been any central instruction to change the way individual governors implement home detention curfew Prison Service

Only prisoners sentenced to less than four years' imprisonment were eligible for tagging.

In addition, many offenders are not allowed early release, including those convicted of sex, child cruelty and racially-aggravated crimes.

Napo assistant general secretary Harry Fletcher has called for increased investment in the probation service "to create more capacity for less serious offenders to be supervised in the community".

A Prison Service spokesman said: "There has certainly not been any central instruction to change the way individual governors implement home detention curfew."