The British foreign secretary, Jeremy Hunt, has proposed a European-led maritime protection force in the Gulf – spurning separate American plans for a US-led force to protect commercial shipping from Iranian interference.

Speaking to MPs, Hunt said the maritime security force would protect crew and cargo from what he described as “wholly illegal acts of state piracy by Iran”.

The proposal has parallels with existing international maritime forces operating in the Gulf as well as EU anti-piracy force operating off the east coast of Africa. However, these missions are protecting shipping largely from non-state actors.

Hunt stressed the UK did not want to join the American campaign of maximum economic pressure since the UK, unlike the US, remained supporters of the Iran nuclear deal signed in 2015. The UK Foreign Office clearly fears a US-led force operating through the strait of Hormuz would be seen as escalatory by Iran and possibly increase the chances of further flashpoints.

In a sign of how the operation has been scrambled together in the past 48 hours, Hunt said he would be speaking to his European allies and others later in the week to discuss how the operation might work.

Hunt admitted the US had first requested the UK to contribute to a US-led maritime protection force on 24 June, leading to a formal request on 30 June.

He said the US plan was helpful and it was important to coordinate, but said he wanted the contribution to the proposed naval protection force to be as broad as possible, implying a number of European countries would not be willing to contribute assets if the force was US-led.

There were signs that his decision, a big geo-strategic call in favour of Europe’s continued support for the nuclear deal, may not prove popular with Atlanticist supporters of the expected incoming prime minister, Boris Johnson. Hunt and Johnson have been vying for the prime-ministership in a contest set to end on Tuesday.

Michael Fallon, the former defence secretary, tipped by some to become Johnson’s foreign secretary, challenged Hunt to declare that it would not make sense for the US to be excluded from the proposed force if they wished to join. After he spoke, another Johnson supporter, Iain Duncan Smith, was picked up on the Commons microphone muttering under his breath that the Americans “are the only ones that have got any assets”.

Hunt conceded some ground. “We would not ... seek to exclude the American navy because they have a vital role for example in the refuelling of our own ships, the communication system, the command and control system, and indeed intelligence support. So we would always operate in partnership.”

Explaining his decision, cleared at a Cobra meeting chaired earlier in the day by Theresa May, Hunt said: “Because freedom of navigation is a vital interest of every nation, we will now seek to put together a European-led maritime protection mission to support safe passage of both crew and cargo in this vital region. We have had constructive discussions with a number of countries in the last 48 hours and will discuss later this week the best way to complement this with recent US proposals in this area.

“The new force will be focused on free navigation, bearing in mind that one-fifth of the world’s oil, a quarter of its liquefied natural gas – and trade worth half a trillion dollars – pass through the strait of Hormuz every year. It will not be part of the US maximum pressure policy on Iran because we remain committed to preserving the Iran nuclear agreement.”

In the meantime, he said the UK had despatched HMS Duncan, a Type 45 destroyer, that was due to reach the area by 29 July.

He had asked “all British-flagged ships to give us notice of any intention to pass through the strait of Hormuz to enable us to offer the best protection we can. We will then advise them as to the safest way to transit, which may involve travelling in a convoy”.

Hunt said the UK was also “strengthening measures to protect ships flying the flags of other countries but which have British crew on board”.

Given the number of UK-flagged ships, he said it is not possible for the Royal Navy to provide escorts for every single ship or indeed eliminate all risks of piracy, but the risks could be substantially reduced if commercial shipping companies cooperated fully with UK government advice, he said.

In a sign of how widely the UK has consulted on the proposals so far, Hunt disclosed he had spoken to the foreign ministers in Germany, France, Finland, Sweden and Denmark.

He also spoke to the US secretary of state, Mike Pompeo, who – probably aware of the UK decision – had earlier told Fox News that it was primarily the UK’s responsibility to look after its own shipping in the Gulf.

Bob Sanguinetti, the chief executive of the UK Chamber of Shipping, expressed concern about the timescale, saying: “While the European maritime mission is a step in the right direction, it is imperative that the UK government continues to protect the interests of British-flagged ships in the Gulf while the mission is assembled. This means formulating a mechanism that will provide confidence and security to allow ships to pass through the straits safely and securely.”