A British warship sped to help a UK-flagged oil tanker as it was seized by Iran, but the frigate was ten minutes too late, it was reported.
The HMS Montrose was dispatched while the Stena Impero was in Omani waters, however it arrived after the tanker had entered Iranian territorial waters.
The Stena Impero had been sailing through the Strait of Hormuz to Saudi Arabia when it was seized by the Iranian Revolutionary Guard on Friday, escalating tensions in the Gulf.
Iran's semi-official Fars news agency claimed on Saturday that the detained tanker was in an accident with an Iranian fishing boat and ignored its distress call.
Two British oil tankers were seized by Iran - the Stena Impero and the Mesdar, a Liberian-flagged vessel which is managed by Norbulk Shipping UK.
The Mesdar was released, but the Swedish-owned Stena Impero, which is registered in the UK, was surrounded by four vessels and a helicopter at about 4pm BST, and taken to a port as Iran claimed it had breached international law.
Stena Bulk said the tanker was in international waters at the time, and no contact had been established since then.
British vessels have been advised to "stay out of the area" of the Strait of Hormuz for an "interim period", a Government spokesman said.
The warning comes after a meeting of the Government's emergency committee Cobra on Friday night, with a spokesman saying the Government remains "deeply concerned about Iran's unacceptable actions".
The spokesman added: "As the Foreign Secretary has said, our response will be considered and robust and there will be serious consequences if the situation is not resolved."
Stena Bulk, which owns the Stena Impero, said the ship was in "full compliance with all navigation and international regulations".
A defence source told The Times that HMS Montrose, which is providing protection to British cargo ships in the Gulf, sped to help the Stena Impero while it was sailing in Omani waters.
However, the warship arrived ten minutes too late - the cargo vessel was in Iranian territorial waters.
Marine tracking data showed the tanker making a sharp turn north towards Qeshm Island, home to a Revolutionary Guards base, after passing through the strait at the mouth of the Gulf.
The ship's tracker was turned off.
The source told The Times Iran appeared to be taking a "very aggressive stance" and was viewed as having been "ready to engage" HMS Montrose if she had arrived in time.
An Iranian official told Fars that the tanker had been taken to Bandar Abbas port and all 23 crew would remain onboard until the end of an investigation.
Erik Hanell, president and chief executive of Stena Bulk, said: "There are 23 seafarers onboard of Indian, Russian, Latvian and Filipino nationality.
"There have been no reported injuries and the safety and welfare of our crew remains our primary focus. We are in close contact with both the UK and Swedish government authorities to resolve this situation and we are liaising closely with our seafarers' families."
The Mesdar veered off course towards the Iranian coast after it was surrounded by 10 speedboats and boarded by armed guards at around 5.30pm on Friday.
The Mesdar's Glasgow-based operator said communication had since been re-established with the ship and the crew were unharmed.
Already strained relations between Iran and the West have become increasingly fraught since the British navy seized Iran's Grace 1 tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria in breach of EU sanctions.
Iran's supreme leader Ayatollah Ali Khamenei called the tanker's seizure an act of "piracy" on Tuesday and warned the UK to expect a response.
Foreign Secretary Jeremy Hunt offered to help release Grace 1 if Iran guaranteed it would not breach sanctions imposed on Bashar Assad's regime.
Last week, the Royal Navy warship frigate HMS Montrose drove off three Iranian vessels which tried to stop the commercial ship British Heritage as it sailed through the Strait of Hormuz.
Meanwhile, a leading authority on British shipping says oil and gas prices will be affected if naval tensions continue in the Strait of Hormuz.
There "is no alternative route in and out of the Gulf" for approximately a fifth of global oil and a third of the world's gas supply, according to Bob Sanguinetti from the UK Chamber of Shipping.
Mr Sanguinetti told the BBC: "If this is to endure then clearly it's going to impact on trade routes, trade patterns and ultimately the price of those goods going through the Strait because they are going to have to be sourced from elsewhere."