Britain has agreed to create a unified naval force with nine European countries to deter future Russian threats from the “open sea border” to the north, the head of the Royal Navy has announced.

Gen Sir Gwyn Jenkins said that despite the ongoing crisis in the Middle East, where the strait of Hormuz remains closed after the US-Israeli war in Iran, “Russia remains the gravest threat to our security”.

In a speech, the first sea lord said the 10 members of the Joint Expeditionary Force (Jef), had signed last week a statement of intent to create a “multinational maritime force” to act as a “complement to Nato”.

It would not include the US, whose president, Donald Trump, has repeatedly criticised the UK for not actively supporting the bombing of Iran, at one point describing the Royal Navy’s aircraft carriers as “toys”.

Military cooperation between the US and the UK is at a low point, with the two countries at odds over the strait of Hormuz. The US has said it wants help forcing the strait and criticised British and French discussions over creating defensive patrols after the war ends, calling them “silly”.

The Jef includes the Netherlands, all five Nordic and the three Baltic states, with the UK as its largest military member. Canada is also considering joining as some Nato members refine their response to rising Russian aggression.

Earlier this month, the UK said that Russian spy submarines had been tracked engaging in what appeared to be covert surveillance of the undersea infrastructure around Britain.

“Russian incursions into our waters has jumped by almost a third in the last two years,” the British naval chief said, adding he considered that the UK had an “open sea border with Russia to the north”.

The aim of the new maritime force, which would be “commanded if necessary” from the UK’s military headquarters in Northwood, north-west London, would be to train and prepare together.

It also would be “designed to fight immediately if required, with real capabilities, real war plans, and real integration”, Jenkins said, though the navy struggled to make a warship available at the start of the war in Iran.

It took more than three weeks after a drone strike on the British Akrotiri base in Cyprus to deploy HMS Dragon into the eastern Mediterranean, and the destroyer later had to dock to fix toilet and water supply problems.

Jenkins said the Middle East crisis had put the navy in the spotlight. “Were we prepared enough? Can we fight today, and if so, with what?” he said, adding that the service had an overall plan of action.

Naval sources have said the availability crisis at the start of the war stemmed from a lack of prior investment and highlighted the impact of cuts made to shipbuilding by previous governments.

Jenkins, a former royal marine and special forces commander, said “uncrewed escort ships” – large sea drones – would sail alongside UK warships in the next two years, as part of an effort to increase military capability but at a lower cost than before.

However, while the UK has threatened to seize Russia-linked “shadow fleet” tankers exporting sanctions-hit oil, it has not done so while other European countries have successfully done so.

Russia has committed frigates to escort vessels under sanctions through the strait of Dover in defiance of the UK warning, made publicly by Keir Starmer on 25 March. Since then, 98 tankers that are subject to sanctions have passed through British waters.