The UK is stepping up its military presence in the Gulf by sending a second warship to the region to protect British commercial oil tankers, the Ministry of Defence has said.

The HMS Duncan, a Type 45 destroyer, will be deployed within days after it completed a course of Nato exercises in the Black Sea with the aim to be in the Gulf region by next week.

The ship will work alongside the Royal Navy’s frigate HMS Montrose and US Gulf allies, but will not participate in Washington’s proposed global maritime coalition to protect shipping in the area.

HMS Duncan was due to have taken over from HMS Montrose, but for a fixed period the two ships will instead work alongside one another.

The news comes after Theresa May said she would begin talks with US authorities on increasing a transatlantic presence in the region following two sets of attacks on oil tankers in the Gulf of Oman, and an attempt by Iranian navy to push a British owned oil ship towards Iranian waters.

The UK already has four mine-hunting vessels stationed in the region, and former naval commanders, including Admiral Lord West, have warned the UK only has eight frigates on patrol.

West’s fears have been echoed by Jeremy Hunt, the foreign secretary, who has renewed his call for UK spending on defence to double.

Hunt said the UK was not seeking to escalate the crisis with Iran and that it was a time for cool heads. “We take maritime security very seriously. It is our primary responsibility to protect British shipping. This is a situation that is changing every hour, but we are reacting to what is happening in a clear and very measured way,” he added.

The MoD said it was seeking to de-escalate the crisis, but conceded that with more than 15 British-owned tankers going through the Gulf each day it was not possible to protect all shipping.

It said: “As part of our long-standing presence in the Gulf, HMS Duncan is deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for pre-planned maintenance and crew change over.

“This will ensure that the UK alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping lane.”

Britain’s already poor relations with Iran deteriorated further last week when the UK seized an Iranian ship, Grace 1, carrying oil destined for Syria, a contract that the UK said was in breach of EU sanctions. Tehran has denied the ship was heading for Syria and threatened to seize a British oil tanker in retaliation, if Grace 1 was not released.

The ship’s captain and chief officer were arrested by the Gibraltarian authorities after the vessel was searched for more than a week. On Saturday the Royal Gibraltar police said the pair and two second officers had been conditionally bailed without charge, while the investigation was continuing and the vessel remained in detention.

Iran has accused the UK of piracy summoning the British ambassador to Tehran, Robert Macaire, four times in the past week. It has suggested the tanker was seized “at the behest of” the US government “in line with America’s hostile policies”.

A spokesman for the Iranian foreign minister, Abbas Mousavi, said: “This is a dangerous game and we advise them not get involved in this game under America’s influence.”

He added: “We ask them again to release the tanker immediately, which will be in all countries’ interest.”

A senior Iranian cleric, the Ayatollah Kazem Sedighi, was reported by state TV as saying: “The British will soon get a slap from the strong arms of our establishment for their daring, reckless and negligent act that will make them regret it for good.”

Is the US nudging Britain into dangerous waters with Iran? Read more

The UK is among the European supporters of the 2015 Iranian nuclear deal, from which Donald Trump withdrew the US last year, reimposing toughened sanctions on Tehran.

When imposing the sanctions in May, the US ordered other countries and companies to halt imports of Iranian oil or face being excluded from the global financial system.

In retaliation, Iran has decreased its commitments to the nuclear pact, in defiance of a warning by European countries.

Timeline Recent tensions in the Gulf Show Hide Tensions between the US and Iran have soared in 2019, with Washington dispatching warships to the Gulf, and Tehran resuming higher uranium enrichment. The UAE says four commercial ships off its eastern coast 'were subjected to sabotage operations'. Yemen's Houthi rebels launch a drone attack on Saudi Arabia, striking a major oil pipeline and taking it out of service. Saudi Arabia subsequently blames Iran for the attack. A rocket lands near the US embassy in the Iraqi capital, Baghdad, without harming anyone. It's not clear who is behind the attack, but after the initial reports, Donald Trump tweets: 'If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!' Saudi Arabia says 26 people were wounded in an attack by Yemen's Houthi rebels on an airport in the kingdom's south-western town of Abha. Two oil tankers near the strategic strait of Hormuz were reportedly attacked in an assault that left one ablaze and adrift. 44 sailors were evacuated from both vessels and the US navy assisted. Iran’s Revolutionary Guards say they have shot down what they called a US 'spy' drone they claim was flying in in the country’s airspace. The US military confirm one of its drones has been taken down, but say it was in international airspace. Donald Trump reportedly gives approval for the US military to launch strikes on Iran in retaliation for the loss of the drone, before pulling back at the last minute. The Iranian and US presidents trade insults, with Hassan Rouhani suggesting that Donald Trump suffered from a “mental disorder” and Trump once more threatening Iran with “obliteration”. Iran summons UK ambassador over an incident off Gibraltar as Royal Marines seize a tanker, Grace 1, the UK suspects of carrying oil to Syria. The International Atomic Energy Agency, the body tasked with verifying Iranian compliance with the terms of the nuclear deal, verifies that Tehran has breached the agreed 3.67% limit for enriched uranium. The UK government says three Iranian boats were warned off by the frigate HMS Montrose after Iranian boats 'attempted to impede' a British oil tanker in strait of Hormuz. Tehran denies involvement. In a major escalation, Iran seizes the Stena Impero, a British-flagged tanker, off its coast. Iranian officials later make it clear that the capture was in retaliation for the capture of the Iranian supertanker Grace 1 earlier in July. Despite US attempts in the courts to prevent it, Gibraltar says it will free oil tanker at centre of the Iran row. Iran gives assurances the oil is not destined for Syria, where selling it would breach international sanctions against Britain accuses Iran of breaching those assurances after Tehran acknowledged the oil had been sold, and the reflagged tanker Adrian Darya 1, previously known as Grace 1, had reached its final destination after the ship was photographed off the coast of Syria.

The US blames Iran for a series of attacks on shipping in the world’s most important oil artery since mid-May, allegations that have been denied by Tehran. The US has also dispatched extra troops to the region to counter what it described as Iranian threats.

Meanwhile Mohamed ElBaradei, a former director general of the International Atomic Energy Agency, asked the UK not to get involved in US plans for military convoys to protect oil tankers in the Gulf.

Accusing the US of behaving like a “bull in a china shop” and taking the world down a dead-end street, he said the confrontational approach of the White House was not going to solve the threat posed to the nuclear deal by the US decision to withdraw from it.

The UK claims it has not yet seen the details of the US proposal for coordinated international naval convoys through the Gulf. This is despite Trump having raised the proposal at the end of last month, before the G20 summit in Japan, emphasising that he wanted to see more Asian countries contributing to the cost of escorting tankers. More than 60% of the oil that goes through the Gulf is heading for Asian markets.

On 28 June, after other attacks on merchant ships, Brian Hook, the US special representative on Iran, travelled to London to meet the secretary general of the International Maritime Organization, Kitack Lim, to press the case for greater coordination. After the meeting Lim said: “[My] primary concern is that merchant vessels, peaceably going about their business, should not be subjected to unprovoked attacks.”

The UK is increasingly caught in a vice, continuing to support the nuclear pact, while Iran slowly reduces its commitments under the deal in protest at Europe’s failure to do more to circumvent the impact of US sanctions on EU firms that want to trade with Iran.

Iran says the central agreement of the nuclear deal was that Tehran would permit close international oversight of its nuclear facilities in return for closer economic ties with the west.

The tensions between the UK and Iran over the oil tankers has added another dimension. The UK does not have sufficient naval assets in the region to protect all British-owned shipping, but it would be wary if the US were effectively in sole command of any such operation.

ElBaradei said: “They [the US] are applying a waterboarding method to Iran, drowning Iran and then saying to them: ‘Let’s have a dialogue without pre-conditions.’”

He said he was horrified by the US policy on Iran. “The new crazies are going out of the way to look for confrontation. This is an opportunity for the British to say that they are standing behind values and principles and not continuing to act as a proxy for the US.”

The former nuclear inspector said if Iran was harassing ships, that was wrong, but he described its plans to enrich uranium beyond the limits set in the deal as “trifling, symbolic and a cry for help”.

• This article was amended on 22 July 2019. An earlier version incorrectly said HMS Duncan was to be deployed to the Gulf after taking part in Nato exercises in the Baltic Sea. These exercises were in the Black Sea.