Former British defence secretary Penny Mordaunt said she had to raise her fears over the growing threat to UK ships in the Arabian Gulf five times before she was granted a ministerial meeting.
Ms Mordaunt led the defence department under Theresa May but said the prime minister did not heed repeated demands for an emergency meeting between May and July.
Eventually she secured the meeting on July 10, with discussions centring on the risk after UK territory Gibraltar seized the Grace 1 supertanker.
The ship, taken by UK marines, was laden with Iranian crude being delivered to Syria in contravention of EU sanctions. It was released in mid-August.
Ms Mordaunt said the decision making process had been "hollowed out", with ministerial oversight marginalised.
"There was a lack of appreciation of the important value of ministers taking political decisions and changing the options that officials had presented," she said.
The following day the Ministry of Defence ordered that the HMS Duncan be sent from the Black Sea to the Gulf. But before it arrived the Iranians had seized the British-flagged Stena Impero.
Speaking to a parliamentary committee, Ms Mordaunt, who was sacked by Boris Johnson when he took over as prime minister, said she believed the Stena Impero made mistakes that contributed to its capture.
The vessel gave just three hours' notice of its transit plans, not the 24 hours that other vessels had observed. It then moved outside its own submitted schedule.
As a result the Royal Navy frigate that tried to provide protection could not reach it in time to stop the Iranians taking the boat.
Parliamentarians were shocked by the revelations and suggested that Mrs May had not properly focused on the demands of her defence minister.
"It was very clear," said Julian Lewis, a Conservative MP who chairs the defence select committee. "You were not given the support you needed."
Lord West, a former head of the British Navy, said the government should known that the Grace 1 could be off-loaded in Syria because its officials were known "liars".
Iran has said it planned to release "within days" the oil tanker it had seized in July.
Foreign ministry spokesman Abbas Mousavi told state television that the necessary steps to set the Stena Impero free were under way.
"And, God willing, the boat will be released in the coming days," Mr Mousavi said.
The seizure of the Stena Impero was seen as a retaliatory move for the Grace 1 seizure.
Now renamed Adrian Darya-1, it is carrying 2 million barrels of oil which it is believed to be discharging at a Syrian port.
Iranian state TV has reported that the ship’s cargo was “sold at sea”.
A spokesman for the Foreign Office called on Iran to fulfil its undertaking on both tankers.
"We continue to call on Iran to immediately release the Stena Impero and her remaining crew, who continue to be illegally detained," it said.
"Iran’s illegal seizure of a ship inside an internationally recognised shipping lane is unacceptable and undermines international law.
"Reports that the Grace 1 is off the coast of Syria are deeply troubling. Iran gave official assurances that Grace 1 would not deliver oil to Syria.
"Breaking those assurances would be a violation of international norms and a morally bankrupt course of action.”