The Co-op has become the latest employer to place workers under AI surveillance with an automated listening technology that rates every phone call some of them make to customers.

In a system described as “oppressive and dystopian” by a whistleblower, Co-op Legal Services is using AI models to record, analyse and award a percentage score for interactions between agents and customers seeking advice about probate, wills and estates.

A model from OpenAI has been trained to assess more than 50 discrete aspects of each phone call and provide pass and fail scores to managers who use the data to analyse employee performance. One worker said the system can mean their work is monitored by AI for several hours a day as they handle inquiries often from recently bereaved customers.

It comes after the Guardian last week revealed Euan Blair’s AI training company, Multiverse, has started using an AI for blanket monitoring of online classroom sessions, which teachers called “remorseless” and “unnerving”, while Multiverse said it was “using AI to improve the experience of learners and customers”. Experts have voiced fears that increasing AI monitoring of workers risks increasing stress.

“You are being watched,” said a Co-op worker who spoke to the Guardian on condition of anonymity and said the AI created a sense of distrust. “It’s different from occasionally being listened to for compliance purposes. [You are] being monitored absolutely every word that you say and every time that you utter something … It just feels really dystopian. It’s the thin end of the wedge.”

The Co-op, which operates supermarkets, undertakers, insurance and legal services companies, said it did not recognise the criticism and said its probate advisers were highly engaged. It said it supported colleagues with AI, which is a “key aspect of our quality assurance processes” and helps ensure “our clients consistently receive empathetic expert guidance”.

The AI monitoring is understood to have been introduced in the last couple of months into the work of dozens of legal services staff. It is used in part to identify ways they can boost their sales performance.

The Co-op said the AI is used solely as a support tool, is not a decision-maker, and helps leaders “support colleagues and continuously improve the experience we provide to clients”.

Trade unions have begun pushing back against the spread of AI monitoring. In February Burger King in the US announced it would be using the technology to track restaurant workers’ customer interactions, and this summer, Meta paused tracking of employees’ computer keystrokes amid data privacy concerns and a staff backlash.

“AI is sold as the solution to every problem, but examples like this make it clear that it can diminish sensible workplace practices and intensify working lives for no reason,” said John Chadfield, the national officer of the Communication Workers Union. “Unaccountable computer systems should not be people’s managers, and these hard-working employees demand far more respect.”

“The office is becoming more like a factory now our performance feedback can be given in a more structured way,” said Hayfa Mohdzaini, a senior policy and practice adviser at the Chartered Institute of Professional Development. “Clearly some people have a problem with it. It will be interesting to see how it develops and how much managers need to adjust their approach.”

This year, the International Labour Organization warned about the “psychosocial” risks of AI monitoring, including increasing work-related stress.

“AI systems are being deployed in workplaces in the absence of a comprehensive regulatory framework,” it said. “Preventing the risk of harm by AI use and operation in this context may not be straightforward.”

Caoilionn Hurley, the managing director of Co-op Life Services, which includes legal services, said: “The efficiency of AI over traditional methods allows us to review all customer conversations to ensure we consistently deliver high standards of service, while enabling more informed support, coaching and development for colleagues … Put simply, our goal is to provide expert support combined with empathy and understanding for every client at what can be a difficult time. AI helps us deliver that more consistently, while human judgment, accountability, care and kindness remain at the heart of every client relationship.”