The UK will convene 35 countries – excluding the US – to explore ways to reopen the strait of Hormuz, the vital shipping route for oil and gas that has been blocked by Iran.

Keir Starmer, the prime minister, said the next phase of discussions in the joint British and French efforts to secure the waterway would be held on Thursday, with Yvette Cooper, the foreign secretary, alongside international leaders.

Donald Trump has said it will be up to other countries to make the strait safe if the US ceases its strikes on Tehran, criticising the lack of backing for his war from European nations.

Starmer said on Wednesday the meeting would bring together 35 countries to “assess all viable diplomatic and political measures we can take to restore freedom of navigation, guarantee the safety of trapped ships and seafarers and to resume the movement of vital commodities”.

No 10 said it would be the first time the countries had convened to discuss a viable plan to reopen the strait. The prime minister said British military planners would meet afterwards “to look at how we can marshal our capabilities and make the strait accessible and safe after the fighting has stopped”.

But Starmer, who convened energy and shipping bosses at No 10 on Monday, said the clear-up would last a long time after the hostilities had ceased. “I do have to level with people on this, this will not be easy,” he said.

“They were clear with me, the primary challenge they face is not one of insurance, but one of safety and security of passage. So, the fact is, we need all of this together – a united front of military strength and diplomatic activity, partnership with industry, so they too can mobilise once the fighting has stopped and, above all, clear and calm leadership. That is what this country is ready to provide.

“Because my guide from the start of this conflict has always been the British national interest. And freedom of navigation in the Middle East is in the British national interest.”

The meeting will convene the countries who signed a joint statement last month. Several more have joined since. They include the UK, France, Germany, Italy and the Netherlands, as well as Australia, Japan, Canada, South Korea, New Zealand, the United Arab Emirates and Nigeria. It commits the countries to a “readiness to contribute to appropriate efforts to ensure safe passage through the strait”.

It is understood the US has not been invited directly to participate in the talks, with the focus on those who signed the joint statement, as well as other European allies and leading maritime and regional players in the region.

About 1,000 ships are stranded by Iran’s partial blockade of the strait in response to the strikes by the US and Israel. Before the conflict, tankers carried about a fifth of the world’s oil and gas supplies through the channel, and about a third of the global fertilisers necessary for half of the world’s food production. Only about 130 ships have made the passage since the war began, the number that would normally pass through every day.

The Ministry of Defence has sent military planners to US Central Command to look at options for getting tankers through the strait.

On Wednesday, Iran’s Islamic Revolutionary Guards Corps said in a statement on state TV that the strait of Hormuz would remain closed to “enemies of this nation” and that it remained under control of its navy.

Trump posted on Wednesday that there would be no ceasefire with Iran until it had relinquished control of the waterway. “We will consider when Hormuz Strait is open, free, and clear. Until then, we are blasting Iran into oblivion or, as they say, back to the Stone Ages!!!” he wrote.