Trials of a £3.5 billion tank fleet being made in Merthyr Tydfil had to be paused recently over a raft of problems, the Ministry of Defence (MoD) has confirmed.

Issues facing the Ajax armoured fighting vehicles reportedly included them being unable to travel safely over 20 miles per hour.

The Daily Telegraph reported that problems with the Ajax tanks – which are supposed to be able to travel up to 40mph – also include an inability to fire cannons on the move.

Citing a UK government report, the newspaper said the speed restrictions had been caused by excessive vibrations in the tanks, and that crews were limited to 90 minutes inside them at a time.

Due to be published next month, the report says the tanks cannot reverse over obstacles more than 20 centimetres high, that personnel must wear noise-cancelling headphones when operating them and undergo ear tests afterwards, and that the Household Cavalry Regiment “cannot conduct effective collective training” in them.

The MoD decided to bring in a new range of tanks in 2010, with 589 vehicles across a range of models ordered in 2014.

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Defence secretary Ben Wallace previously said the Ajax fleet would "provide the British Army with improved flexibility, manoeuvrability and battle-winning capability and advantage. Its advanced reconnaissance systems will allow British soldiers to see things first, weigh up their options and take decisive action before their adversaries."

But the programme has been plagued by problems and is four years behind schedule, The Telegraph said, adding only 14 of the turretless Ares model tanks, made by US firm General Dynamics, had been delivered.

General Dynamics' UK sites include factories in Merthyr and Oakdale. The MoD said in a statement its training on “Ajax variants” had now recommenced “with appropriate safety measures in place”.

“We are committed to the Ajax programme which will form a key component in the Army’s modernised warfighting division, with current plans for Initial Operating Capability scheduled for summer 2021,” an MoD Spokesperson said in a statement.

“The MoD can confirm that some training on the Ajax family of vehicles was paused as a precautionary measure.

“This is a normal measure for the demonstration phase of projects; an investigation, incorporating trials, is being carried out jointly with the manufacturer. It is inappropriate for us to comment further at this time.

“The health and safety of our personnel is of the utmost importance and we are committed to providing a safe working environment.”

Additional reporting by Trevor Marshallsea, PA