Mark Fullbrook, the prime minister’s closest adviser, was questioned as a witness in a US Department of Justice (DoJ) and FBI investigation into the matter.
In April, the FBI approached the UK’s National Crime Agency and the Metropolitan Police to help secure Mr Fullbrook’s attendance for a formal interview, The Sunday Times first reported.
The US investigators are looking into allegations that a London-based businessman tried to bribe the governor of Puerto Rico, a United States territory.
Julio Martin Herrera Velutini, a UK Conservative Party donor and the owner of the Puerto Rican bank Bancredito, is accused of bribing Puerto Rica’s governor at the time, Wanda Vazquez Garced.
Federal prosecutors claim Mr Herrera wanted Ms Vazquez, a Trump-supporting Republican, to sack her financial regulator, who was investigating his bank, and replace them with someone of his choosing.
UK Electoral Commission records show that a company founded by Mr Herrera also donated £650,604 to the Conservative Party between December 2019 and June 2022.
Mr Fullbrook, who masterminded Ms Truss’s leadership campaign, was interviewed by US authorities because of his work for, and role at, CT Group, an Australian-British lobbying firm founded by Lynton Crosby.
Court records in the United States allege that the alleged bribe was paid through CT Group. Mr Fullbrook was employed as the company’s chief global project officer, and prosecutors claim that he flew to Puerto Rico to meet with Vazquez Garced.
Prosecutors are reported to be reading through hundreds of sensitive emails from Mr Fullbrook relating to his work in the Puerto Rican capital, San Juan.
Mr Fullbrook strongly denies any wrongdoing. A representative said: “Our client was not involved in the illegal conduct being alleged and is not being investigated in relation to it.
“He is merely a witness to it, and has and is fully cooperating with the relevant US law enforcement authorities. Any allegation that he is unfit to hold the position of chief of staff is wholly untrue.”
The representative stressed that the prime minister’s adviser “had absolutely no knowledge of any alleged bribery or corruption when CT Group contracted to provide services to Julio Herrera Velutini”.
They also said that Mr Fullbrook did not carry out any work for Ms Vazquez Garced, the politician allegedly being bribed.
Last month, the DoJ charged Ms Vazquez and Mr Herrera with crimes that could see each face up to 20 years in prison. Both deny any wrongdoing.
Commenting on the investigation, assistant attorney general Kenneth A Polite Jr said: “The alleged bribery scheme rose to the highest levels of the Puerto Rican government, threatening public trust in our electoral processes and institutions of governance.
UK news in pictures
“The Department of Justice is committed to holding accountable those who wrongly believe there is one rule of law for the powerful and another for the powerless. No one is above the rule of law.”
US attorney W Stephen Muldrow said: “The criminal actions of the defendants in this case strike a blow to the heart of our democracy and further erode the confidence of our citizens in their institutions of governance.
“Our resolve to bring to justice those entrusted by the public to serve with integrity and who violate that trust remains steadfast. Equally steadfast is our resolve to prosecute those who seek to use their wealth and power to enrich themselves at the expense of honest government. I commend the dedication and hard work of the law enforcement personnel and prosecutors in this case, as well as those individuals willing to come forward and cooperate.”
Mr Fullbrook is married to former Tory MP Lorraine Fullbrook, who was ennobled and given a life seat in the House of Lords by Boris Johnson in 2019.
Downing Street has not yet commented on the allegations.