Labour has called on the Conservatives to consider returning money from one of their largest donors, amid allegations that his oil business was involved in a Syria bribery scandal.
Oil executive Ayman Asfari and his wife, Sawsan, have given £623,000 to the Conservatives since 2009, and the businessman is a member of the Leader's Group, the elite group of donors who in return for £50,000 a year are invited to join David Cameron and other senior figures from the Conservative party at dinners.
Asfari is chief executive of Petrofac, while he and his family are some of Petrofac's largest shareholders and among the richest 200 people in the UK, according to the Sunday Times rich list.
But a Labour frontbencher told BuzzFeed News the Tories should reconsider their relationship with Asfari after an investigation by Australian newspaper The Age and the Huffington Post alleged that Asfari's Petrofac business employed a company that paid bribes to the Syrian government in the late 2000s for the rights to build oil plants in the country.
"The prime minister must be able show that these donations are beyond reproach," said shadow cabinet member Jon Ashworth MP.
The Age and the Huffington Post said their joint investigation was based on a huge cache of leaked emails from a company called Unaoil.
The investigation quotes emails from the Monaco-based firm, run by the millionaire Ahsani family, that show Unaoil worked for Western oil firms, bribing officials to help win these clients government-funded projects on a huge scale.
Based on these emails, The Age branded Unaoil "the company that bribed the world", alleging the corrupt payments involved millions of pounds and covered many countries.
On Thursday, Monaco's legal officials raided Unaoil's office with the UK's Serious Fraud Office to investigate a "vast corruption scandal with international ramifications involving many foreign companies operating in the oil sector".
According to the newspaper, some firms that engaged Unaoil were apparently unaware of the firm's possible methods, some turned a "blind eye", and some were fully conscious of possible misbehaviour.
In a leaked 2008 email, Cyrus Ahsani of Unaoil calls Petrofac "by far our largest customer", and discusses having a face-to-face meeting with Asfari.
A Petrofac document from 2010 states: "Since 1987 Petrofac has enjoyed a long and close association with the oil and gas industry in Syria."
In 2008, three years before the Syrian uprising that sparked the civil war, Petrofac won a contract to build the $454 million Jihir plant from the Syrian-state-controlled Hayan Petroleum Company. In 2010 Petrofac said they enjoyed "supportive relationships with regulatory authorities" in Syria, including around the building of the Jihir plant.
But the Age papers allege that Unaoil paid €630,000 to Nissan Khouchaba, a businessman with close ties to the regime of Syrian president Bashar al-Assad, on behalf of Petrofac. The payment was nominally for a "market survey" but The Age said the documents suggested they payments were payments to a "middleman".
Asfari has personally supported moderate opposition to al-Assad's regime since the outbreak of the country's civil war in 2011.
The Age said it had emails from Khouchaba to a senior Petrofac executive, Peter Warner, discussing the payments. Petrofac's 2007 report said Warner had "overall divisional responsibility for the identification of potential opportunities; management of bid submissions" for the whole company.
Warner provided many references for Unaoil so it could pass audits showing the firm was "ethical", the media outlets alleged. Warner left Petrofac in 2012 and in 2014 joined the board of directors of Unaoil.
According to Conservative party records, Asfari went to at least two Leader's Group dinners in 2015, along with Cameron, George Osborne, foreign secretary Phillip Hammond, and other ministers.
BuzzFeed News made repeated requests to the Conservative party for a response, but had not received one by time of publication.
"These allegations about Petrofac are very worrying," Labour's Jon Ashworth said. "There are now serious questions to be answered about the propriety of the donations to the Conservative party from its chief executive, Ayman Asfari.
"Given the privileged access enjoyed by Ayman Asfari to the prime minister and senior members of the government through the exclusive Leader's Group, the prime minister must be able show that these donations are beyond reproach. If he cannot, then he should return the money immediately."
BuzzFeed News approached Petrofac for its reponse to the allegations of bribery involving Unaoil.
A spokesperson said: "Petrofac takes any suggestion of improper conduct very seriously and we are looking into the allegations made. Petrofac does not condone any activities that contravene our Code of Conduct, which includes strict anti-bribery and corruption standards."
A spokesperson for the Serious Fraud Office said it was "aware of the allegations, but can neither confirm nor deny our interest in the matter".