Liam Fox, the UK trade secretary, and the US Treasury secretary, Steven Mnuchin, joined key European partners in pulling out of a major economic forum in Saudi Arabia nicknamed Davos in the desert, in response to the alleged murder of the journalist Jamal Khashoggi.

Reports of Khashoggi’s gruesome murder at the hands of a gang of 15 men with links to the Saudi royal court have already led to many western media firms and bankers pulling out, and the political lead from Fox and Mnuchin is likely to accelerate the boycott of the Future Investment Initiative (FII) conference in Riyadh next week.

Khashoggi, a critic of the Saudi regime, was apparently tortured to death and dismembered in the Saudi consulate in Istanbul, having entered the building to obtain a divorce certificate.

Jamal Khashoggi. Photograph: Virginia Mayo/AP

The British withdrawal was announced soon after the French finance minister, Bruno Le Maire, and his Dutch counterpart, Wopke Hoekstra, confirmed they were withdrawing from the event starting on 23 October. The UK made its decision in co-ordination with its allies, but without knowing the precise American decision.

A UK government spokesman said Fox had decided it was not the right time to attend the conference and explicitly tied the decision to the events in Turkey.

“The UK remains very concerned about Jamal Khashoggi’s disappearance,” the spokesman said. “We encourage Turkish-Saudi collaboration and look forward to the kingdom of Saudi Arabia conducting a thorough, credible, transparent and prompt investigation, as announced,. Those bearing responsibility for his disappearance must be held to account.”

The announcement of the Mnuchin pull-out came via Twitter roughly four hours after the French and British announcements. The US decision was taken after the US secretary of state, Mike Pompeo, consulted with Donald Trump on Thursday.

Pompeo had said no US action would be taken until a joint Turkish-Saudi investigation was completed, but Mnuchin’s withdrawal suggests the US has enough information to implicate Saudi Arabia, if not yet Crown Prince Mohammed bin Salman. Pompeo said they needed to give Saudis “a few more days” to complete a thorough and transparent inquiry.

Pompeo said he had told the Saudis he took Khashoggi’s disappearance “very seriously”. Pompeo flew to Riyadh and to Turkey to discuss the crisis this week, but has made few comments on the progress of the investigation. He said on Thursday that the Saudis had promised a thorough investigation.

The conference organisers have taken down the list of expected speakers, as they face a mass boycott by media sponsors and businesses. Frédéric Oudéa, the director general of Société Générale, Jean Lemierre, chairman of BNP Paribas, and Patrice Caine, the chief executive of Thales, have all pulled out.

Also on Thursday, the Fox Business Network became the latest media organisation to withdraw from the event. It had been a media sponsor.

Two other sponsors, CNBC and Bloomberg, as well as CNN, the Financial Times, the New York Times and the Economist had already announced they would not participate.

In the past week the UK has been co-ordinating its position on Saudi Arabia with France, and Germany, but it is unusual for the British to make such a symbolic show of condemnation of Riyadh. It is also striking that Downing Street took the decision in advance of any announcement from the US.

The US is already locked in conflict with Iran, and is determined not to open a second front in the Middle East by clashing with its greatest ally in the region. The US is due, from 5 November, to impose sanctions on any country or company importing oil from Iran, and may feel it cannot risk counter-sanctions by the Saudis at a time of such potential oil market uncertainty.

But Trump is under pressure from Republican senators to take a stand over Khashoggi’s disappearance, and there is a risk the issue could come in to play in next month’s mid-term elections.

The Russian president, Vladimir Putin, put the issue in Trump’s court by pointing out that Khashoggi was a US resident. He said Russia had no reason to sanction Saudi at least until clear evidence is produced.

Other senior figures have already dropped out of the event, including the head of the International Monetary Fund, Christine Lagarde, and representatives of major tech and media companies such as Google, Uber, the New York Times and the Financial Times.

The former international development secretary Andrew Mitchell had been among those calling for Fox to boycott the event.

Crown Prince Mohammed bin Salman attends the 2017 Future Investment Initiative (FII) conference in Riyadh. Photograph: Fayez Nureldine/AFP/Getty Images

“As matters stand it would be inconceivable that a British minister could attend this Saudi economic function next week,” he told the Times.

The foreign secretary, Jeremy Hunt, signed a statement of G7 foreign ministers on Wednesday demanding answers on Khashoggi’s disappearance.

“We remain very troubled by the disappearance of prominent Saudi journalist Jamal Khashoggi,” the statement said, calling for a “thorough, credible, transparent, and prompt investigation, as announced.”

Allan Hogarth, Amnesty International UK’s head of policy and government affairs, welcomed the cancellation, calling it “a recognition of the fact that cheerleading for business in Saudi Arabia when the country is accused of murdering a journalist is simply beyond the pale”.

Hogarth said conducting business with Saudi Arabia was “tainted well before the shocking disappearance of Jamal Khashoggi”.

“The UK needs to seriously re-appraise its entire relationship with Saudi Arabia, breaking free of overly cosy relations which have seen ministers unwilling to publicly criticise human rights abuses in the country.”