Probation officers have voted to reject a 5% pay rise and refuse overtime from 23 November, citing mounting pressure from the government’s prisoner early release scheme on a service struggling with high caseloads and staff shortages.

Members of Napo, the probation service union, could also take strike action should the union “deem that option to be necessary”. The vote sets the service on course for industrial action later this month, with a strike remaining a possibility rather than a confirmed next step.

Napo general secretary Ian Lawrence said the service was “already in crisis” before the early release scheme came into force. That scheme relies on probation staff to monitor prisoners in the community after they have been freed early.

Responding to the vote, a Ministry of Justice spokesperson said the government had put forward a “strong offer” on pay and was recruiting an extra “1,300 officers to significantly cut workloads”.

The spokesperson added that the government had “robust contingency plans in place” for any industrial action, to ensure offenders “are still subject to the same rigorous supervision”.

Probation service unions have warned that officers already have too many cases to handle. They have also warned that the rate at which staff are leaving the profession has increased, adding to concerns about staffing and workloads.

The union said the service had come under renewed pressure in recent months because of the early release scheme. The government introduced it to manage prison capacity as jails came dangerously close to being full.

On 1 October, 700 eligible offenders who had originally been sentenced to up to 19 months were released. Two further groups, each comprising 650 prisoners, were released on 13 October and 10 November.

Another nine groups are expected to leave prison over the next seven months. Inmates who received longer sentences will be included in the later cohorts, with some 250 prisoners serving more than 12 years expected to be released on 9 June.

Lawrence said staff had tried “desperately” to implement the early release schemes introduced by both the Labour government and the previous Conservative government. But they had been asked to carry out that work with “inadequate staff and a lack of resources”, he said.

The government has said electronic tags, alongside intervention by probation staff, will help to manage prisoners who are freed early under the scheme.

The Probation Service uses electronic monitoring to track compliance with curfews and the conditions attached to a court or prison order. Most of those now eligible for early release require tagging, making electronic monitoring a central part of the arrangements for supervising them outside prison.

But a September report by His Majesty’s Inspectorate of Probation (HMIP) found that almost half of the electronic tags used to track offenders’ movements were not fitted on time.