David Cameron’s activities at the scandal-hit Greensill Capital finance company are a “matter of interest” in a wider investigation by the Serious Fraud Office, the Guardian understands. The SFO, which investigates and prosecutes fraud, bribery and corruption in the UK, has questioned interview subjects about the UK foreign secretary’s involvement with the now-defunct company, sources claim. The Guardian understands that Cameron’s activities have been discussed in sensitive interviews with witnesses in the long-running SFO investigation. A spokesperson for Cameron declined to answer specific questions about the investigation and his involvement with Greensill, but said that the foreign secretary had not personally had “any contact” with the SFO. Senior figures who worked at Greensill have been questioned as part of the SFO’s broader inquiry into “suspected fraud, fraudulent trading and money laundering” at companies within the Gupta Family Group Alliance (GFG) steel, energy and commodities empire, an investigation announced in 2021. GFG, owned by the steel tycoon Sanjeev Gupta, has consistently denied any wrongdoing. Greensill was the main financial backer of the loose industrial grouping of energy, steel and commodities assets founded by Gupta. Witnesses have been questioned on the role Cameron played in promoting Greensill to investors and his engagement with and promotion of GFG, the Guardian understands. Greensill specialised in supply chain finance, a way of advancing cash to companies, and raised huge sums from banks including Credit Suisse before collapsing in 2021. The SFO said it could not comment on the details of an ongoing investigation, which is also examining GFG’s “financing arrangements with Greensill Capital”, which employed Cameron between 2018 and 2021. The finance company’s founder, Lex Greensill, a banker with farming roots in rural Australia, was given access to the heart of government during Cameron’s tenure at No 10. In 2012, Greensill shared business cards showing that he had an official No 10 email address and was working under the title “senior adviser” in the prime minister’s office. Greensill then hired the former prime minister as an adviser in 2018. Cameron was widely criticised for using contacts garnered while prime minister to personally lobby politicians and senior civil servants on behalf of Greensill when he worked for it. He sent WhatsApp messages to the then chancellor, Rishi Sunak, and text messages to a former top Treasury civil servant, Sir Tom Scholar, during the height of the Covid pandemic. The foreign secretary’s lobbying showed a “significant lack of judgment”, according to a parliamentary inquiry by the Treasury committee in 2021. Cameron, who rejoined the government and was handed a peerage late last year, has previously said he played “no role in the decisions to extend credit, or the terms on which such credit was extended, to GFG or any other customer”. During their inquiry into the scandal, MPs on the Treasury committee said: “We question Cameron’s judgment in relation to his lobbying on behalf of Greensill.” They added: “Cameron appears to have relied heavily on the board of Greensill as a guarantee of its propriety and financial health, when arguably he should have taken a broader and more enquiring assessment of the business. There were signals available to Cameron at the time when he was lobbying the Treasury and others which might have led him to a more restrained approach.” His lobbying efforts, which included directly approaching senior figures at the Bank of England as well as in Whitehall and Westminster, emphasised the need to support British industrial jobs, noting Greensill’s backing for GFG. Greensill and its backing for GFG has been subject to regulatory and potentially criminal inquiries in Germany, Switzerland and the UK. Greensill told British MPs in 2021: “At no point would my firm have engaged in financing receivables that we knew to be fraudulent.” He declined to comment on GFG specifically, although Cameron has called GFG and Greensill relationship “symbiotic”. A spokesperson for the foreign minister said: “Lord Cameron has not had any contact with the SFO.” The Labour party called on Cameron in December to reveal the full extent of dealings with Gupta. Nick Smith, the shadow deputy leader of the House of Commons, has written to Cameron to ask him to share all correspondence and documents linked to the Greensill/GFG relationship and provide a full account to parliament of what he shared about the two organisations with Sir Laurie Magnus, the prime minister’s ethics adviser, upon his appointment as foreign secretary.