Liz Truss’s chief of staff still owns a stake in Sir Lynton Crosby’s lobbying company despite his Downing Street role, meaning he could benefit financially from the firm’s work for corporate clients. Mark Fullbrook, who is in charge of the prime minister’s political operation, co-founded the British arm of the lobbying business CT Group with Crosby. Its clients have included the tobacco company Philip Morris, the Saudi Arabian government, and the mining firm Glencore. Fullbrook left CT Group earlier this year but continues to own 10% of its shares, a potential conflict of interest which has not been publicly disclosed since he joined Downing Street. His stake in the lobbying company could be worth millions of pounds in the event of a sale, while he would be entitled to a share of any dividends paid by the business as a result of its work for corporate clients. He spent a decade working for the company, which was well known for its close links to successive Conservative governments. During Fullbrook’s time as a director of CT Group it ran a global campaign to promote the burning of coal, paid for by the major miner Glencore. The company also took millions of pounds from the Saudi Arabian government for work in the UK, despite Saudi’s history of human rights abuses. Other clients included Russian oligarchs who wanted to improve their reputations in the west. A spokesperson for CT Group said it was wrong to suggest that the prime minister’s chief of staff owning a stake in one of the UK’s most prominent lobbying businesses created a conflict of interest: “While Mr Fullbrook is in his current position, he has no control, influence over nor indeed knowledge of the group’s business activities.” The spokesperson said that Fullbrook had no entitlement to payment of dividends “arising while he is in his current position”. They added: “This was purposely done before he took his current position to ensure there was no conflict of interest, actual or perceived, and no financial benefit. To claim otherwise would be completely wrong and deliberately misleading.” While at CT Group, Fullbrook worked closely with Boris Johnson – who gave a seat in the House of Lords to Fullbrook’s wife in 2019 – and oversaw Zac Goldsmith’s losing 2016 London mayoral campaign, which was dogged by accusations of racism. The prime minister’s chief of staff is already the subject of press interest after attempting to have his publicly funded Downing Street salary paid through his separate lobbying company. He was also recently questioned as a witness by the FBI as part of an investigation into an alleged political bribery scandal in Puerto Rico involving CT Group and a Conservative party donor. During his brief stint running his own company, Fullbrook’s clients included the disputed government of Libya and a medical supply firm that won hundreds of thousands of pounds’ worth of government contracts to supply PPE to the NHS during the Covid emergency. Sources at CT Group told the Guardian it was well known among staff that Fullbrook continued to hold a substantial financial stake in the company. A No 10 spokesperson declined to comment, while CT Group said any communications with government officials were declared in the appropriate way. Downing Street advisers are only required to state any potential conflicts of interest on an annual basis. Fullbrook has not yet been asked to make such a declaration, meaning there is no suggestion of any wrongdoing. CT Group was also involved in a series of unbranded Facebook pages that appeared to be a grassroots campaign for a hard Brexit in 2019. The campaign, which involved spending up to £1m of money from an unknown source on Facebook adverts, helped push Theresa May out of power and sow the seeds for the election of Boris Johnson as prime minister. The company has previously set up a network of websites that appeared to be independent news outlets but were actually lobbying fronts paid for by clients. Reuben Solomon, the CT Group employee who administered some of the apparent news pages, is now a special adviser to Truss and runs Downing Street’s digital campaigns. Fullbrook was central to CT Group, serving as a director and running the British arm of the business – then known as CTF Partners. He would deal directly with clients while Crosby was on secondment to Tory election campaigns. Crosby, an Australian, ran the Conservatives’ general election campaigns in 2005, 2015 and 2017, and Johnson’s campaigns for London mayor in 2008 and 2012. He was knighted in 2015 for political services. One former colleague said: “He [Fullbrook] and Lynton worked closely, sharing an office … Mark was obviously in thrall to Lynton. He seemed to think he was the luckiest man alive that Lynton chose him as the partner that would get him into the UK political scene.” Although Crosby and Fullbrook continue to share a financial interest in CT Group, there have been reports of a schism between the pair. According to the Times, Fullbrook recently told new Downing Street colleagues that he was not Crosby’s protege – and instead it was the Australian election strategist who learned from him. A CT Group spokesperson denied there was a split: “Mark and Lynton are peers and remain close friends. This is a personal relationship based on knowing and working together for years and can in no way be interpreted as anything other than that.”