Labour has demanded answers after it emerged Liz Truss’s chief of staff Mark Fullbrook was questioned as a witness as part of an FBI inquiry into alleged bribery in Puerto Rico.

In a letter to Cabinet Secretary Simon Case, Labour deputy leader Angela Rayner urged him to clarify when he first became aware of the allegations.

She also pressed Mr Case on when the information was shared with the Prime Minister and whether Mr Fullbrook made any declaration of his involvement in the investigation as a witness when he took up his role in Downing Street.

Liz Truss visit to US for the United Nations General Assembly
Prime Minister Liz Truss (Toby Melville/PA)

The FBI investigation relates to allegations that financier and Tory donor Julio Herrera Velutini promised to help the former governor of Puerto Rico get re-elected if she dismissed an official investigating a bank he owned there.

He has denied the charges against him.

Mr Velutini is alleged to have paid CT Group, a political consultancy firm in which Mr Fullbrook was a senior figure, 300,000 US dollars (£260,000) for work intended to help Wanda Vazquez Garced’s ultimately unsuccessful re-election campaign in 2020.

Mr Fullbrook was only treated by the FBI as a witness and his spokesperson said he “complies with all laws and regulations in any jurisdiction… and is confident that he has done so in this matter”.

Ms Rayner said she wrote to Mr Case “in the absence of an independent adviser on ministers’ interests following the resignation of Lord Geidt” in June.

Conservative former prime minister Boris Johnson did not replace him and Ms Truss suggested she may not appoint an ethics adviser.

In her letter, Ms Rayner said the reports about Mr Fullbrook were “incredibly alarming revelations which the public will rightly want clarity on”.

“The allegations about the new Prime Minister’s most senior adviser once again prompt questions about this Government’s ethics, values and basic standards of decency,” she wrote.

“Public trust is already hanging by a thread.

“These revelations have hallmarks of a Government that continues to disregard basic standards of decency, and an utter disregard for British national security. It increasingly looks like more of the same under the new Prime Minister.

Ministers’ standards
Labour deputy leader Angela Rayner (Dominic Lipinski/PA)

“In the interest of transparency and out of respect for our democratic institutions, I urge you to clarify when you were first made aware of these allegations.

“Importantly, at what stage was this knowledge shared with the Prime Minister? Were any declarations made by Mr Fullbrook about his involvement in this investigation when he took on the position of chief of staff?”

Ms Rayner later accused Ms Truss of preserving an “ethical vacuum at the heart of Downing Street”, adding: “While Labour has a plan to clean up politics with our Independent Ethics and Integrity Commission, the Conservatives under Liz Truss show all the signs of letting sleaze fester in Number 10.”

Ms Truss has given her “full support” to Mr Fullbrook.

A Downing Street spokeswoman said the Prime Minister stands 100% behind him and “he has her full support”.

Asked whether Mr Fullbrook informed her about the case before she appointed him chief of staff, Ms Truss told reporters on the plane on her way to New York: “All staff being appointed by the Government go through a proper process. And that is exactly what has happened with Mark Fullbrook.”

Pressed further, she added: “We have a propriety and ethics team which make sure that everybody is properly going through the process.”

A spokesman for Mr Fullbrook said: “As has been made repeatedly clear, Mr Fullbrook is committed to and complies with all laws and regulations in any jurisdiction in which he works and is confident that he has done so in this matter.

“Indeed, Mark Fullbrook is a witness in this matter and has fully, completely and voluntarily engaged with the US authorities in this matter, as he would always do in any circumstance in which his assistance is sought by authorities.

“The work was engaged only by Mr Herrera and only to conduct opinion research for him and no one else.

“Mr Fullbrook never did any work for, nor presented any research findings to, the governor or her campaign. There has been no engagement since.

“Mr Fullbrook understands that there are active legal proceedings against other individuals and entities. It would therefore be inappropriate to comment further.”