The multiplying allegations about high-level Saudi military involvement in the abduction or murder of the journalist Jamal Khashoggi is placing an unprecedented strain on the close security and trade relationship between the UK and Saudi Arabia, which has been a pillar of British foreign policy.

The British foreign secretary, Jeremy Hunt, has in the past been willing to defend the relationship, saying it has prevented bombs from going off on the streets of London. It has also kept the Treasury coffers fuller.

The red-carpet welcome for Mohammed bin Salman on his three-day visit to London in March was based on that longstanding relationship but also fortified by the hope that the crown prince was a reforming figure sidelining a conservative religious police. The visit led to 13 memoranda of understanding and the promise of £65bn of investment in the UK over the next decade.

Privately, British diplomats have never harboured any illusions that Bin Salman’s social reforms, such as allowing women to drive, presage greater political liberty. But in aiming to diversify the Saudi economy through his Vision 2030 plan, the crown prince was at least giving post-Brexit Britain the chance to extend the relationship beyond military hardware into construction, transport and education.

The Khashoggi episode threatens to make that idea less attractive and, incidentally, render the millions spent by Saudis on western PR agents largely wasted.

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Although it is hardly a secret that Saudi Arabia jails journalists, such a brazen state-endorsed act in a foreign country would cross a line. In the US and the UK, politicians and diplomats normally classed as friends of the Gulf kingdoms – such as Bob Corker, the Republican chair of the Senate foreign affairs committee – appear shocked.

For the UK, in the middle of a démarche over extra-jurisdictional attempted killings by Russia in Salisbury, to turn a blind eye to reports alleging such a brutal act would be perceived as high hypocrisy.

The dilemma is even more acute as Britain has chosen to support the Turkish president, Recep Tayyip Erdoğan, since the failed military coup in Turkey in 2016. Turkey may not be a champion of press freedom but it will expect other foreign states to condemn any violation of its sovereignty.

The Foreign Office has had its differences with the Saudis over the Iran nuclear deal but knows how little Saudi Arabia welcomes direct criticism. A single tweet in August by Global Affairs Canada, calling for the release of a female rights protester, led to the recall of the Saudi ambassador, 8,300 Saudi graduates being withdrawn from the country, a freeze on investment and the cancellation of all flights from Saudi to Canada.

Adel al-Jubeir, the Saudi foreign minister, accused Canada of treating the kingdom like a banana republic. “You can talk to us about human rights anytime you want, we’d be happy to have that conversation like we do with all our allies. But lecturing us? No way. Not going to happen,” he said. Spain and Germany have also been warned.

The big question is whether the episode will prompt a wider rethink about the value of the Saudi relationship to the UK. The UK argues proximity secures influence, a point it repeatedly makes in defending its support for the Saudi-led bombing campaign in Yemen. But it is reluctant to discuss the alliance more broadly. A British official due to defend the relationship at a Chatham House seminar last month withdrew at the last minute.

Two recent external studies have urged the UK to rethink the relationship’s value. The Policy Institute at Kings College accepts that the Gulf remains the lifeblood of the UK arms export industry but argues that the overall value of the Saudi Arabia relationship is vastly overstated.

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“UK trade exports in goods and services to Saudi Arabia totalled £6.2bn in 2016, but this represents a mere 1% of the UK’s total export value in 2016, which amounted to £547bn. Imports from Saudi Arabia account for 0.3% of all of those into the UK, or £2bn out of a total of £590bn in 2016. Arms sales constitute 0.004% of total revenue to the Treasury in 2016,” it said.

A second study, Anglo-Arabia, published last month by Dr David Wearing, argues that the relationship is based largely on Britain’s role as a world leader in financial services, with Gulf petrodollars and sovereign wealth funds financing the UK current account deficit. It says such a deal is inherently unstable.

Britain may yet be able to avert a rethink if the evidence of Saudi government involvement just falls short of overwhelming. A FBI investigation proposed by the US might sow enough doubt.

Yet Theresa May’s self-declared task for “Global Britain” has been set high: “To defend the rules-based international order against irresponsible states that seek to erode it.” Global Britain is facing a first and exacting test.