Some prisoners released early as part of efforts to ease overcrowding have not been fitted with electronic tracking devices, despite it being a condition of their release, the BBC has been told.

Several offenders released on licence said they had not been fitted with an ankle tag – with one describing it as “a disaster waiting to happen”.

One probation officer said the delays in them being fitted appeared to be due to a shortage of tags, while the Ministry of Justice said it was due to a backlog of former prisoners.

The MoJ blamed security contractor Serco – which manages the prison tagging system – for the delays. Serco said it was working to reduce the numbers waiting for tags.

Prisons minister Lord James Timpson said the delays to tagging were “completely unacceptable” and called what he described as an “urgent meeting with Serco leaders” on Thursday evening.

He said they “accepted the need for immediate action and personally committed to me to eliminate the backlog” – which, Lord Timpson added, he expected would “be done at pace over the coming weeks”.

“All offenders continue to be closely monitored by the Probation Service with stringent conditions and we are prioritising the tagging of domestic abusers on licence to ensure the safety of victims.”

More than 1,700 prisoners were let out early in England and Wales last week as part of the government’s scheme to address overcrowding in prisons.

The justice secretary said this had prevented the prison system from reaching maximum capacity and “a total breakdown of law and order”.

But the scheme has been criticised as risking public safety, while not all victims were made aware of their offender’s early release date.

Offenders jailed for violent offences with sentences of at least four years, sex offenders and domestic abusers were not eligible for early release, the government has said.

Many of those let out last week were released on licence.

In some cases, that means having to wear an ankle tag so that their movements can be monitored to check they are obeying a curfew or following other restrictions.

Those who are not tagged are able to act without being monitored – increasing the risk of reoffending and danger to the public.

One offender released last week told the BBC: “They should have tagged me but they haven’t and it means I can go where I choose.

“I don’t want to end up back inside so I’m sticking to the curfew, but I know people who aren’t doing that because they’ve not been tagged. It’s a disaster waiting to happen.”

In his case, he has been told to stay at home from 19:00 to 07:00.

Another offender released last week said: “I have to stay indoors till I get fitted, which means I can’t leave and it’s doing my head in – but I don’t want to get recalled into jail, so I’d rather do that but it would be easier if I was tagged. It’s a disgrace.”

Three probation officers spoke to BBC News on condition of anonymity.

One, who said they believed the issue was down to a shortage of tags, said: “We’re hearing the same story from several staff saying that people haven’t been tagged yet and it’s going to cause problems.

“Seems like people were released without the conditions on which they were released on being properly ready, and that could potentially be dangerous.”

Tagging or Home Detention Curfew (HDC) is a scheme which allows some people to be released early from custody if they have a suitable address to go to.

If someone is released on HDC, they have rules to follow about where they can go and what time they have to be back at home.

It allows the authorities to monitor the movements of an offender to dissuade them from reoffending.

According to Martin Jones, the chief inspector of probation, about a third of those released early last week are expected to reoffend.

The tagging system is operated by Electronic Monitoring Services (EMS), an organisation which is managed by Serco under a contract with the MoJ.

An MoJ spokesperson said: “Tagging is an important part of our strategy to keep victims safe and cut reoffending, and there are currently record numbers of offenders wearing tags.

“We are holding Serco to account to address delays in fitting some offenders with tags, and will apply financial penalties against the company if this is not resolved quickly.

“While this issue is ongoing, we have prioritised tagging domestic-abuse offenders to make sure their licence conditions, such as staying away from their victims, are strictly followed.”

A Serco spokesperson said: “Since we took over the electronic monitoring contract in May we have been working hard to reduce the number of people waiting to have a tag fitted.

“We work closely with the MoJ and the probation service to fit tags swiftly and prioritise cases based on risk profiles.

“Where an individual is not at home when we call to fit a tag the time taken can be longer. We prioritise making another visit so that people are tagged as soon as possible.”