Soldiers had to be given medical treatment for hearing problems this summer during final testing of the British army’s new Ajax armoured vehicle, whose introduction has been delayed for several years amid concerns about deafness.

The model, which costs £5.5bn for 589 vehicles, was nevertheless classified as fit for deployment in September. An investigation concluded there were “no systemic issues” – but there remain health concerns among the troops involved.

One source said “people are still being hurt”. It is understood that a small number of soldiers reported noise and vibration concerns and had to be processed at a military medical centre.

Luke Pollard, a junior defence minister, said on Wednesday Ajax’s noise and vibration problems were “firmly in the past” at a launch event for the armoured vehicle, which has been formally accepted into army service.

“If it were not safe, we would not be putting it in the hands of our frontline troops,” Pollard said. “I’ve been reassured from the top of the army down to the folks that work on the platform that it is safe.”

But subsequently, when the Ministry of Defence (MoD) was asked about issues raised by soldiers, a spokesperson acknowledged there had been problems. “Where concerns were raised over the summer, these were investigated by a safety team and no systemic issues were found,” they said.

The MoD would not say exactly how many were treated after noise complaints, though it said there had been no reports of tinnitus motion sickness. Care took place in a primary GP-style care facility, it added.

Concerns about deafness from military service have plagued the British army for decades. In September, thousands of UK veterans launched a legal claim against 3M, saying its combat earplugs, used in Afghanistan and elsewhere, were not effective. The US manufacturer says its product worked if used properly.

This week journalists toured the Ajax manufacturing site, a General Dynamics factory in Merthyr Tydfil, and were transported on the 7.6-metre-long tracked vehicle, which weighs about 40 tonnes and has a top speed of just over 40mph (65km/h).

A meter on a mobile phone showed a reading of 90 decibels from a few metres away. Long or repeated exposure at sounds above 85 decibels can cause hearing loss, and employers must provide protection if that level is exceeded, according to the Health and Safety Executive.

The crew and those riding onboard were provided with a dual layer of protection – earplugs and headphones – and the MoD said safety had improved when this system was introduced. Soldiers on or in a moving Ajax could move for six hours and still be three-quarters below safety limits, defence sources said.

Ajax has been delayed by years, most recently because of concerns about noise and vibration. It was first intended to be introduced in 2017, then in 2020 and 2021, but trials had to be halted in June 2021 because of safety concerns.

A review, published in December 2021, revealed that 11 people out of 310 who tested Ajax had “long-term restrictions on noise exposure recommended” by a medical assessment, while 17 were under specialist outpatient care.

At the time, a defence minister in the then Conservative government said the episode showed the army did not place as high a value on safety as cost and value for money.