The former defence secretary Penny Mordaunt has revealed that she had to ask five times before Theresa May would agree to a meeting of Britain’s crisis Cobra committee to discuss the Iranian threat to Gulf shipping.

Mordaunt said she had become so frustrated after her three requests were ignored that she wrote to the then prime minister in mid June to ensure there was “an audit trail” as she warned the UK was “very likely facing having a ship taken”.

Giving evidence to the defence select committee on Monday, Mordaunt said she believed the Cobra system had become “hollowed out” under May – but despite that, it was necessary to hold a “granular meeting where we thrashed those issues out”.

Cobra meetings, named after the room in the Cabinet Office which they are held, are convened to discuss specific crises. Mordaunt said she believed the system had become less well-resourced than it had been when she was a junior minister under David Cameron’s premiership.

“There was a great difference in my experience of the Cobra set up between my time as minister for the armed forces and my time as secretary of state. It had been hollowed out.” Later she added: “The backroom operation was not there.”

Mordaunt’s fourth request – made on 19 June – was rejected again, this time in writing. An official letter she received said: “The decision to call a ministerial Cobra is for the PM; events to date have been judged not to warrant this.”

That came after four oil tankers were attacked near the strait of Hormuz on 12 May, and two other tankers were sabotaged by limpet mines on 13 June. The US and UK blamed Iran for the incidents, although Tehran denied responsibility.

Tensions continued to escalate, with US President Donald Trump coming within minutes of launching an airstrike on Iran on 21 June, in retaliation for the downing of a American drone a few days earlier.

Mordaunt said that she asked May again, and a Cobra meeting was finally convened on 10 July – six days after an Iranian oil tanker, the Grace 1, had been seized by Royal Marines when it passed through Gibraltar – but before a British flagged ship, the Stena Impero, was taken by Iran on 19 July.

The former defence secretary, who was only in post 85 days before she was sacked by Boris Johnson when he became prime minister, was giving evidence to a defence select committee inquiry into the Gulf shipping crisis. MPs on the committee expressed surprise that it had taken so long to convene a Cobra meeting.

The Stena Impero and its 23 crew were successfully seized by Iran’s Revolutionary Guards following a mix-up over its location, Mordaunt said. The vessel did not take the course it had said it would, meaning that navy patrol ship the HMS Montrose was too far away to protect it.

“I don’t believe that the Royal Navy or the MoD dropped the ball,” Mordaunt added.