Trials of the £3.2 billion Ajax armoured vehicle programme have been halted for a second time over complaints about noise affecting the crew.
The Ministry of Defence (MoD) said “all Ajax trials have been suspended” following renewed concern about the issue.
Defence minister Jeremy Quin acknowledged the programme – which could eventually cost taxpayers up to £5.5 billion – was “troubled”.
The vehicles, built by General Dynamics, are billed as “fully digital” and the “next generation of armoured vehicles”.
But trials were halted for four months to March after crews reportedly suffered nausea, swollen joints and tinnitus.
Testing resumed with “mitigations” in place but has now been halted again.
Mr Quin said: “This long-running troubled programme requires ongoing intense work by our industrial partners and ourselves to ensure its delivery.
“In achieving this, the safety of our personnel will always come first.”
The MoD’s top mandarin has launched a thorough safety investigation, working with General Dynamics, which is expected to conclude at the end of July.
A spokesman for the firm said: “General Dynamics Land Systems-UK is confident in its vehicle design and stand behind Ajax.
“The MoD has informed us that the pause is temporary and precautionary only.”
Officials insisted the MoD remained committed to the Ajax programme.
Mr Quin faced questions about Ajax from MPs on June 8 and said the MoD was looking at two headsets to help address the noise problems, although he admitted that would not get to the “root cause” of the issues, which he suggested could be mechanical or electronic.