US Treasury Secretary Steven Mnuchin and UK International Trade Secretary Liam Fox have pulled out of an investment conference in Saudi Arabia.
Their move comes amid allegations the country was behind the killing of Saudi journalist Jamal Khashoggi.
Mr Khashoggi has not been seen since entering the Saudi consulate in Istanbul on 2 October, where Turkish officials allege he was murdered.
Saudi Arabia, which denies the killing, allowed investigators inside overnight.
The Dutch and French finance ministers, as well as several other politicians and business leaders, have also said they are pulling out of the summit.
However, a number of major businesses - including Goldman Sachs, Pepsi and EDF - are still intending to go despite growing pressure for a boycott.
Just met with @realDonaldTrump and @SecPompeo and we have decided, I will not be participating in the Future Investment Initiative summit in Saudi Arabia.
— Steven Mnuchin (@stevenmnuchin1) October 18, 2018
Mr Mnuchin announced he would pull out of the summit following a discussion with President Donald Trump and Secretary of State Mike Pompeo.
Mr Pompeo went to Riyadh earlier this week to discuss the disappearance with Saudi King Salman.
He said he had been assured that Saudi Arabia would investigate the matter, and that he had told Mr Trump to give the country "a few more days" before deciding whether to take action.
Meanwhile, several high-profile human rights groups have demanded that Turkey ask the UN to investigate the possible killing of Mr Khashoggi.
Amnesty International, Human Rights Watch, Reporters Without Borders and the Committee to Protect Journalists, said that a "one-sided" investigation by Saudi Arabia was not good enough.
What is the summit about?
The Future Investment Initiative summit will take place from 23 to 25 October in the Saudi capital Riyadh.
It is being hosted by the kingdom's Crown Prince Mohamed bin Salman to promote his reform agenda.
His plan for economic and social modernisation, called Vision 2030, aims to reduce Saudi Arabia's dependence on oil and provide new jobs.
As a result, Mr Mnuchin's decision to boycott the summit is seen as a substantial blow to the prince's agenda, correspondents say.
What happened to Jamal Khashoggi?
It is not clear. Mr Khashoggi was last seen entering the Saudi consulate in Istanbul on 2 October, to pick up paperwork that would allow him to marry his fiancée Hatice Cengiz.
Turkish officials believe Mr Khashoggi was killed by a team of Saudi agents inside the consulate, with his body then removed.
Saudi Arabia has denied the claims, and initially insisted Mr Khashoggi had freely left the embassy.
It says reports on Mr Khashoggi's death are "completely false and baseless" and that it is "open to co-operation" to find out what happened.
Is there any evidence?
The Turkish authorities say they have audio and video evidence of the killing - although these have not been made public.
Turkish media with close links to the government have published gruesome details on the alleged audio, saying screams, and the voice of the consul, Mohammed al-Otaibi, could be heard in the recording.
Yeni Safak, which is close to the government, quotes him as telling alleged Saudi agents sent to Istanbul: "Do this outside. You're going to get me in trouble."
Meanwhile, Turkish media say they have identified a 15-member team of suspected Saudi agents who flew into and out of Istanbul on the day of the disappearance.
However, Saudi Arabia says reports on Mr Khashoggi's death are "completely false and baseless" and that it is "open to co-operation" to find out what happened.
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On Wednesday and Thursday, Turkish investigators spent almost nine hours searching the Saudi consul's residence, before moving on to the consulate itself about 200m (650ft) away, according to Reuters news agency.
Several vehicles with Saudi diplomatic number plates were filmed by CCTV cameras moving from the consulate to the residence just under two hours after Mr Khashoggi entered the consulate.
Source: bbc