Sir Keir Starmer has turned to Government lawyers to assess the legality of intervening in the Strait of Hormuz. Donald Trump has repeatedly called on allies, including the UK, to join US-led efforts to reopen the vital shipping route. The President said Britain’s initial response to the Iran conflict had put its status as a "great ally" at risk, adding on Tuesday that the two countries had enjoyed a "tremendous long-term relationship" until Sir Keir became Prime Minister. The Prime Minister has not ruled out providing military support but warned it would be "difficult" to justify under international law. TRENDING Stories Videos Your Say A Government source told The State of It, a political podcast by The Times, that lawyers had been asked to consider whether intervention in the Strait of Hormuz - through which around 20 per cent of global oil flows - could be justified on the basis of self-defence. Some officials fear any military action would require a UN resolution, which could be vetoed by China and Russia. A Government source suggested intervention in the strait was currently "too hot" and that "defensive" operations could not take place while the US-Israeli bombing campaign continued. They added: "Once the offensive attacks and retaliation are over, you can look at supporting the safe opening of the area for shipping." A US diplomatic source said the UK had "hamstrung itself" by setting "clear red lines" it refuses to cross. The source warned that by declaring the initial US-Israeli strikes on Iran illegal, Britain had limited its own scope for military involvement. Dominic Grieve, the former Conservative attorney general, said there was a "legitimate" legal basis for keeping the waterway open. He said the Strait of Hormuz is an "international waterway", adding that Gulf states and "other users of the straits are entitled to exercise the right to legitimate and proportionate self-defence". LATEST ON THE PRIME MINISTER: Keir Starmer set to refuse delay calls over King's US visit despite Donald Trump tensions Keir Starmer accused of 'killing special relationship' with US and striking 'wrong balance' on Iran Keir Starmer risks fresh clash with Donald Trump after ignoring President's Iran plea However, he said the situation had been complicated by US and Israeli strikes on Iran. "Iran would doubtless argue that as using the straits partly involves navigation through their territorial waters they have to close them in light of the attack on them to protect themselves, although this argument looks weak on the facts," he said. Dan Tomlinson, a Treasury minister, told Sky News the situation was "dangerous", with civilian vessels coming under fire from Iranian forces. The UK has allowed US jets to operate from RAF Fairford in Gloucestershire, with those missions deemed lawful under "collective self-defence" to protect British bases and allies. It is understood Britain has placed limits on US bombers using the base, including restrictions on targets and operations. Even if the UK agrees to support efforts to reopen the strait, its naval capacity is limited, with a number of ageing vessels undergoing maintenance. So far, Britain has deployed a single warship to the region - HMS Dragon - which arrived at Gibraltar on Tuesday morning. GB News understands the Type 45 destroyer will not reach Cyprus to defend RAF Akrotiri for at least another week. Our Standards: The GB News Editorial Charter