Media playback is unsupported on your device Media caption Footage released by Iran's Revolutionary Guard-affiliated Fars news agency appears to show Stena Impero being seized

It is set to be a testing week for British diplomacy and for Britain's new prime minister.

The new man at No 10 will inherit a fully-fledged international crisis at a time when Britain's traditional diplomatic ties are under strain like never before.

Brexit has, to say the least, complicated our relationship with our European allies and there are significant differences with the US over the fate of the Iran nuclear agreement.

The strains with the US, provoked by the leaking of blunt comments from the UK ambassador to Washington about the Trump administration's performance (he subsequently resigned), have been accentuated by President Trump's recent rhetorical attacks against four female opposition politicians in his own country.

His critics regard these remarks as racist and many politicians here have judged them, at the very least, as unacceptable.

So this is the diplomatic landscape in which the UK is embarking upon an effort to seek leverage against Tehran.

Diplomatic pressure - action at the UN or tough economic sanctions - requires the building of a coalition.

And it should be said at the outset that neither Brexit nor policy differences with Washington necessarily exclude a common front.

'At loggerheads'

Think back to the collective action taken against Moscow in the wake of the murder of a British woman by Russian agents in Salisbury in 2018.

The US, Nato and Britain's European allies all expelled Russian diplomats in an impressive show of solidarity.

It was a high point for Theresa May's foreign policy.

But will the same solidarity be shown in Britain's quarrel with Tehran?

Image copyright Stena Bulk

France and Germany have given London rhetorical support. President Trump is standing beside his British ally.

But the US and the EU are fundamentally at loggerheads over the fate of the nuclear deal with Iran and what many European capitals see as a thinly disguised US policy that seeks regime change in Tehran.

Indeed the failure of the US effort to gain wider support for its planned maritime security operation in the Gulf - this has been circulating for weeks with few takers - suggests that many governments are deeply reluctant to get involved in anything that looks like a coalition ranged against Iran.

Some governments may also think that Britain - perhaps encouraged by senior officials in the Trump administration - has to some extent brought this crisis on itself.

I previously questioned the decision-making that led up to the seizure of the tanker, Grace 1, carrying an Iranian cargo off Gibraltar.

Was sufficient attention given to Tehran's likely response? Who actually signed off on this decision?

Was this just a failure of "joined-up" thinking or was Britain, as the Spanish government has suggested, following a US steer?

Many of these issues are likely to be raised in Parliament over the coming week as will the light that this episode shines on the state of Britain's naval capabilities.

Here again is the same question; were the correct decisions made? With Iran openly threatening British-flagged shipping did we send the right signal to Tehran?

And if the limitations were largely due to defence cuts, then the size of Britain's surface fleet is surely going to be an issue for the new government.

'Embarrassing'

But with British-flagged vessels keeping clear of the Gulf for now that is not the most pressing concern. The immediate issue is to secure the return of the Stena Impero and its crew.

The Iranians may see a simple exchange of vessels as one answer.

That is presumably why they seized the British tanker in the first place. But London insists that, unlike Iran, it had international law on its side.

An exchange - however it is dressed up - will be embarrassing.

A complication is that the Grace 1's fate is now bound up with legal process in Gibraltar and that will presumably have to run its course before the vessel can potentially go on its way, albeit to a non-sanctioned destination.

And what of the US role in all of this?

The row between London and Tehran is but one element in the wider tensions in the region prompted by the Trump administration's decision to abandon the nuclear deal with Tehran and to re-impose punitive sanctions.

Iran is gradually breaching the various limits and restrictions set by the nuclear agreement.

The Europeans - Britain included - are desperately trying to keep it alive, or at the very least to delay its demise for as long as possible.

This unfolding story is being punctuated by attacks on shipping in the Gulf and the shooting down of drones by both the Iranians and the US Navy - though Iran denies that one of its drones was lost.

At any point a miscalculation could lead to a shooting war.

This is the context in which the new British government must now seek to negotiate the release of its ship.

The likely next prime minister - Boris Johnson - prides himself on his close ties with Mr Trump.

But will this be an advantage given Washington's very different strategic goals?

He has also done much to frustrate Britain's EU allies. He will now have a very difficult course to steer.