The public is “at direct risk” from unsupervised ex-offenders because probation officers in England and Wales are being asked to cope with excessive workloads, a union has said.

As ministers prepare to release and monitor tens of thousands more prisoners this autumn, Napo’s executive has declared for the first time that it has no confidence in managers at the probation service.

In a worrying development for the government, the union is threatening to launch industrial action in three months’ time unless members receive increased support and pay.

The motion comes at a crucial time for the government’s plans to relieve pressure on the criminal justice system. From September, ministers will embark on the biggest expansion of tagging in British history so that up to 40,000 former offenders will be monitored by tags and overseen by probation officers – a 40% increase from the 28,000 currently on tags.

Last year, an official watchdog warned that the probation service had too few staff with too little experience and training, which it said left members of the public at risk. The public accounts committee found that longstanding staff shortages had left probation staff dealing with “excessive and unmanageable workloads”, with officers working at 126% of capacity for several years in some areas.

Tania Bassett, a Napo national official, said probation officers were unable to cope with the growing number of ex-offenders they were being asked to supervise, and many more people were ending up on the street.

“Excessive workloads and staff burnout poses a direct risk to the public with staff being unable to effectively manage the risk of their clients in the community,” she said. “Added to this is the shortage of accommodation, which will result in more people being homeless and therefore more likely to reoffend.”

Managers were trying to get rid of a tool that measures the workload each probation officer is being asked to cope with, a move that would hide the magnitude of the tasks they were being asked to perform, Bassett said.

“The loss of a workload measurement tool will leave staff, including managers, unable to see their workloads and therefore unable to evidence that they are overworked,” she said.

The Prison Service met only 26% of its targets for timeliness of appointments and delivery of services in 2024-25, down from 50% in 2022, according to the National Audit Office. The Ministry of Justice said that between 2023 and 2025, 31% of target probation appointments did not take place.

The MoJ said earlier this year that it would recruit 1,300 extra probation officers in the next year as part of a £700m investment by 2029, which included £100m for the tagging expansion by the end of this parliament. The department said “proximity monitoring technology” for domestic abusers and stalkers would be tested in a £5m pilot.

The union’s executive has voted for a motion that says a failure to address persistently high vacancy and staff sickness rates and removing a tool that measures workloads means that “the current position is untenable and cannot continue”.

It says: “[HM Prison and Probation Service’s] leadership has demonstrably failed in its duty of care to the workforce of the Probation Service, and this represents a reckless disregard for our welfare and professional integrity as well as the safety of our communities.”

James Timpson, the prisons minister, told MPs last week that the probation service was under severe pressure after disclosing that staff were each managing an average of 32 ex-offenders.

“It’s running too hot … we inherited a system that was broken, and we’re putting it all back together again. It’s going to take time,” he told the justice select committee.

A Ministry of Justice spokesperson said: “We remain committed to working closely with trade unions to ensure our staff continue to get the support they need to cut crime and protect the public. We have full confidence in Probation Service leadership to deliver the necessary changes and improvements.”