The Department for Transport has issued new guidance to all British-flagged commercial ships to go to a heightened state of security in the Strait of Hormuz, Sky News understands.

The guidance – raising the level of alert to critical – was issued in recent days in response to concern about an increased threat from Iran in the Gulf, two maritime industry sources said.

British-flagged ships have been advised to keep alert for Iranian patrol boats being aggressive towards them, one of the sources said.

It comes as police in Gibraltar arrested the captain and the chief officer of the Grace 1, the Iranian supertanker which was boarded by Royal Marines amid suspicion it was taking crude oil to Syria, in breach of EU sanctions.

Police say they have been interviewed but not charged. Documents and electronic devices have been seized.

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 of Hormuz during daylight hours and travelling at a faster pace than usual to keep safe.

A Department for Transport spokeswoman said: The Department for Transport, as competent authority, regularly provides Security Advice to UK and Red Ensign Group Shipping on how they should operate in areas of high risk.

US President Donald Trump tweeted on Thursday afternoon, quoting a talk show host who said: It seems that the Brits and the French and the Germans are coming around to the idea that the Iranians are up to no good.

It comes after three Iranian boats tried to stop a British oil tanker in the Strait.

HMS Montrose, a British warship operating in the area, then gave a warning and deployed its helicopter, a wildcat, which circled the Iranians, before their boats withdrew.

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.

However, Iran’s Revolutionary Guard denied claims of a confrontation, saying: There were no clashes with alien boats, especially English boats.

Tensions have been rising since Royal Marines helped seize the Iranian-flagged tanker off Gibraltar.

The capture of Grace 1 prompted Iranian President Hassan Rouhani to warn the UK of consequences.

You [Britain] are the initiator of insecurity and you will realise the consequences later, Rouhani said after a cabinet meeting on Wednesday, according to the semi-official news agency Tasnim.

Brigadier General Amir Hatami also warned the UK its actions will not go without a response.

The US has been leading a push for allies and other tankers that use the Strait of Hormuz to increase their maritime security in the region, safeguarding strategic waters off Iran and Yemen.

It alleges that Tehran was behind attacks on six oil tankers in the area.

Relations between the two countries have fractured since Mr Trump pulled the US out of a nuclear deal with Iran and re-imposed sanctions.

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(c) Sky News 2019: UK ships on ‘heightened security’ as crew of detained Iran vessel arrested