As tensions rose in the region, sources stressed that the Astute-class submarine would perform a purely defensive role, using its sophisticated covert electronic intelligence gathering equipment to safeguard British and international shipping. Last night Iran increased its provocation by publishing images of the Iranian National Guard Corps’ elite Quds Force abseiling from helicopters to board the tanker. It is a mirror of images released two weeks ago by Britain showing Royal Marines descending on Grace 1, an Iranian tanker suspected of carrying oil to Syria.

Foreign Secretary Jeremy Hunt said UK vessels “must and will be protected”. During an emergency call with his Iranian counterpart, Mohammad Javad Zarif, Mr Hunt said he “expressed extreme disappointment that having assured me last Saturday Iran wanted to de-escalate the situation they have behaved in the opposite way”.

He added: “This has to be about actions, not words, if we are to find a way through. British shipping must and will be protected.”

The 7,400-ton submarine, one of Britain’s newest, will be able to plot movements and voice transmissions from 200 miles away.

Its electronic warfare suite can intercept and download all conversations with specialist linguists onboard. They can evaluate the transmission and report back to naval headquarters, Sources say it will focus its attention on the Iranian port of Bandar Abbas, and the flotilla of so-called midget submarines which pose an increasing threat to international shipping along the Strait of Hormuz.

“We are sending a boat – it may well already be heading for the region,’ said a senior Royal Navy source last night.

“Its role is a covert intelligence posture, simply gathering information to support the planned escort convoys of tankers.

“The Astute-class commands a significant electronic warfare capability and does not actually need to be sat in the Gulf to be effective.”

Ambush, the second of the Royal Navy’s potent new Astute Class attack submarines (Image: Royal Navy)

Last night Defence Secretary Penny Mordaunt confirmed that a Royal Navy frigate was just an hour away when Iranian forces took control of the British-flagged Stena Impero in what she described as a “hostile act”. Mr Hunt rejected claims by Tehran that it had performed a “tit-for-tat” act to the British impounding of the Grace 1 supertanker.

“Nothing could be further from the truth. Grace 1 was detained legally in Gibraltarian waters because it was carrying oil against EU sanctions to Syria,” said Hunt.

“The Stena Impero was seized in Omani waters in clear contravention of international law. It was then forced to sail into Iran. This is totally and utterly unacceptable.”

The flare-up comes just a week since Mr Hunt attempted to increase support for the Iran nuclear deal, which saw Tehran earn more than £100billion for promises not to develop nuclear weapons, and which has been on the verge of collapse since the US withdrew support. Senior shipping sources rejected Iranian claims that the Stena Impero which, it has now emerged was carrying no oil, had collided with a fishing boat and had been seized as part of an investigation.

“There is absolutely no credible evidence to support these claims, just as Iran’s earlier position that the vessel had been in Iranian waters bears no link to reality,” said the source.

Last night, former foreign secretary Sir Malcolm Rifkind said the time had come for multinational convoys to protect shipping through the region.

Britain has already increased its maritime presence. HMS Kent, a Type 23 Duke class frigate, will sail from Portsmouth in early September boosting the UK Royal Navy force to nine.

HMS Montrose is off to the Gulf (Image: AFP/Getty Images)

The Navy support ship RFA Wave Knight last night arrived off Gibraltar, en route to join HMS Montrose and a squadron of mine clearance vessels also heading for the Gulf.

And four Royal Navy mine hunters, already in the Gulf on a three-year deployment, have now been further tasked with escort duties.

On Friday intelligence intercepts picked up orders by Islamic Revolutionary Guard Corps (IRGC) commanders to increase heavy artillery within shelling range of the Stena Impero, and construct a new “holding centre”, sources said last night.

Iran is thought to have 2,000 fast attack craft. But it is midget submarines that are the biggest threat, said former Royal Navy Commander Tom Sharpe.

“The Yono midget submarine is a particular menace. Often lurking just below the surface in the middle of the traffic separation system [a maritime traffic-management route-system] they are armed with a couple of heavyweight torpedoes,” said Commander Sharpe.

“These will kill a frigate and possibly even a carrier. There are always a couple at sea and they are hard to track and even harder to defeat.”

Last night former international development secretary Priti Patel said that British attempts had failed to deliver a hoped-for normalisation of diplomatic relations, adding: “We are being played.”

Defence Secretary Penny Mordaunt (Image: David Mirzoeff/PA Wire)

Ms Patel, now a member of the foreign affairs select committee, said: “The level of hostility that we’ve been seeing from the Iranian regime over a period of time should concern us.

“Despite efforts by Britain there is normalisation whatsoever. These are the techniques and tactics that have been associated with the Iranian regime for decades. “We are seeing the essential international rules-based system jeopardised. We are being played.

“While our response must be measured, it is surely time to have more uncomfortable discussions about where we go from here.”

James Rogers at the Henry Jackson Society, which is a British foreign policy think tank, said: “Britain has aligned itself with the EU on the nuclear deal and Iran.

“We are trying to chart a course between the EU line and placate the US at the same time. But Iran is trying to push us off this tightrope and, largely, they are succeeding.

“The timing for these acts is significant. The UK appears weak because of domestic issues; there will shortly be a new Government, and the current one is about to leave.”

He added: “The issue now is whether the EU framework for dealing with Iran is the right one that will work. These latest actions are clear evidence that it’s not working.

“Allowing it to be targeted without responding merely compounds our weakness and compounds that perception around the world.

Foreign Secretary Jeremy Hunt (Image: Aaron Chown/PA Wire)

“Our next government must reconsider its position.We need to free the tanker, prevent additional vessels being taken or derailed from their journey and this requires larger presence from Royal Navy and potentially the use of convoys. This is about freedom of navigation.”

COMMENT BY TOM TUGENDHAT MP

We must stand up to this bully

For 200 years the Royal Navy kept sea lanes open.We guaranteed trade could pass safely through straits like those at the opening of the Persian Gulf.

We stationed ships not just for ourselves but to see international trade flourish.We succeeded beyond our wildest dreams.

Today’s commercial shipping carries goods around the world at prices lower than any time in history.We have enabled a global economy. But today it is under threat.

The seizing of a British ship by the Iranian Revolutionary Guard Corps is a reminder that rules are always tested by those who think they can break them.

Like bullies everywhere, they are encouraged by weakness and sense it in the West today.

Footage of Iranian commandos storming the seized vessel (Image: NC)

They are testing us and we must not be found wanting.

Our resolve, and that of our allies, must be strong against a group that has spread death to Syria and Israel, corrupted and undermined efforts for peace in Iraq and Yemen, and is trying to export violent Islamic revolution around the region.

That means working together.We need to stand with our allies and not give our enemies any reason to doubt our commitment to each other and the international rules we built together.

Of course that means the United States, but much more than that, today it means India and China.

More of the energy shipped through the Straits of Hormuz heads east than west as those growing economies depend on foreign reserves so they too should play their part in defending the supply routes.

If we don’t have the rule of law we will be reduced to the rule of force and, as we have learned over centuries, that would see us all lose.

Today, more than ever, we need to make sure politics isn’t just a history lesson for slow learners.

Tom Tugendhat MP is Foreign Affairs Committee Chairman