Britain has “run down the navy too much”, according to foreign secretary Jeremy Hunt.
His words come as tensions continue to rise in the Gulf after a Royal Navy warship deterred Iranian patrol boats attempting to impede a British tanker sailing through the Strait of Hormuz.
Writing in The Daily Telegraph, Mr Hunt said: “Three vessels from a foreign military power tried to seize a British ship conducting its rightful business.
“The simplicity of these words belies the incredible menace behind Iran’s actions. Not for the first time, Britain’s interests were defended by the Royal Navy.”
Mr Hunt, a Tory leadership hopeful and son of an admiral, said he would add more warships to the fleet if he was chosen to be the next prime minister.
He has vowed to increase defence spending from 2% of GDP to 2.5%, a cost of around £12bn more a year in 2023, and more jets for the navy’s new aircraft carriers could be included.
“When you look at this week’s events it shows that in recent decades we have run down the navy too much,” he said.
“Our current commitment is for 19 destroyers and frigates, supported by excellent offshore patrol vessels.
“If I become prime minister, I will review this commitment as part of a wider look at our defence capability.”
Mr Hunt’s rival Boris Johnson was not as clear with his promises on the issue, saying that it was “very important that UK vessels should be able to use international shipping lanes”.
After Thursday’s incident in the strait, which is a vital passage for the world’s oil supplies, a government spokesman said: “We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”
Sky News also understands British ships have been warned to keep alert for Iranian patrol boats “being aggressive towards them”.
Some 15 to 30 British-flagged tankers and other large ships are in the Persian Gulf on a daily basis. Between one and three pass through the Strait of Hormuz every 24 hours.
Iran’s Revolutionary Guard denied claims of a confrontation, saying: “There were no clashes with alien boats, especially English boats.”
A shipping industry source said his vessels are increasing the number of watch keepers on deck while close to Iran, trying to move through the strait during daylight hours and travelling at a faster pace than usual to keep safe.
Earlier this month an Iranian supertanker – the Grace 1 – was boarded by Royal Marines amid suspicion it was taking crude oil to Syria, in breach of EU sanctions.
On Thursday police in Gibraltar arrested the captain and the chief officer of the vessel.
Iranian President Hassan Rouhani warned the UK of “consequences” after the seizing of the Grace 1.
Meanwhile, the US is pushing allies to increase their maritime security in the region, safeguarding strategic waters off Iran and Yemen.
Relations between the US and Iran have fractured since Mr Trump pulled the US out of a nuclear deal with Iran and re-imposed sanctions.