The UK Ministry of Defence was alerted to problems with its Ajax armoured vehicle programme 11 months before it suspended trials over fears the vehicles were causing injuries to troops, a minister has admitted.

Experts say the problems surrounding the Ajax are so serious the government should consider cancelling the £5.5bn deal to buy 589 of the vehicles, which are expected to increase battlefield surveillance using high-tech digital sensors, and replace them with a smaller alternative.

The MoD ordered the vehicles from US contractor General Dynamics in 2014. But the project has been beset by noise and excessive vibration, prompting a four-month suspension of military personnel training with the tank in November last year.

Responding to a parliamentary question, defence procurement minister Jeremy Quin said this week that soldiers had first reported vibration problems during trials on prototypes at the “end of 2019”.

Further noise and vibration issues were reported in July 2020 and in September a report by medical staff “raised the possibility of noise injuries,” Quin said in a statement. As a result, the MoD commissioned “in-ear assessments” and trials on the Ajax programme were suspended on November 6.

The MoD has already spent nearly £3.5bn on the vehicles and 14 have been delivered. Current testing restrictions limit the time spent in them to 90 minutes and mandate a maximum speed of 20mph. A leaked report by the Infrastructure Project Authority, which reports to the Cabinet Office, warned that “successful delivery of the programme to time, cost and quality appears to be unachievable”.

“There are major issues which, at this stage, do not appear to be manageable or resolvable within the current business case,” the report added.

Ben Wallace, defence secretary, told the Financial Times on Tuesday that he had commissioned “more work” into how the programme could be rectified and said he had met army personnel twice in the past two weeks to discuss the issue.

“I think that what’s important . . . is that I satisfy myself that [the project] I’ve inherited is fixable, and that we protect the interests of the men and women we are going to put in the back of this thing,” he said, adding that any costs for solving the problems would be met by the contractor.

However, Tobias Ellwood, the Conservative MP who chairs the House of Commons defence select committee, raised more fundamental concerns: at 43 tonnes, the Ajax is too heavy to be carried by RAF aircraft, which he described as a “strategic error”.

“In my view it is coming to that point where we should consider cancelling [Ajax] and look into procuring a far smaller vehicle,” Ellwood said. “There’s no doubt that the equipment on it is state of the art, but unfortunately it’s very expensive, very heavy and the noise that it makes is phenomenal.”

The vehicles were due to become fully operational at the end of this month, but this is now expected to be delayed until September.

Ministers announced earlier this year that the planned upgrade to Warrior infantry fighting vehicles would be cancelled and a third of the army’s Challenger II tanks axed as part of a sweeping digital modernisation programme. Uncertainty and delays over Ajax will potentially create a gap in land warfare capability.

Francis Tusa, editor of Defence Analysis, said the implications were serious. “If the Ajax programme collapses — and there’s every chance that it will — then that’s a stake in the heart of the British army, and it will be out of high-end warfighting for five to eight years while they stand up a replacement,” Tusa said. “The UK will have to write a sick note to Nato explaining the problem.”

The MoD said the army had taken “immediate steps” to mitigate the risk to hearing damage after “a small number of soldiers engaged in Ajax trials reported noise issues”. Protective measures include reducing crew time in the vehicle and providing improved hearing protection.

“When investigations found definitive concerns, trials were halted,” the spokesman added.

General Dynamics said a “small number of remaining issues” were being “reviewed and closed out” in partnership with the British army and the MoD.