Labour has demanded answers on how much Liz Truss knew about her No 10 chief of staff Mark Fullbrook being questioned as a witness as part of an FBI inquiry into alleged bribery.
Deputy leader Angela Rayner has written to cabinet secretary Simon Case, urging him to clarify when he first became aware of the allegations.
She pressed Mr Case on when the information was shared with Ms Truss, and whether Mr Fullbrook declared his involvement in the probe as a witness when she appointed him her chief of staff.
The FBI investigation relates to allegations that Conservative party 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.
Mr Velutini – who has denied the charges – is alleged to have paid CT Group, a political consultancy firm in which Mr Fullbrook was a senior figure around £260,000 for work intended to help the governor’s ultimately unsuccessful re-election campaign.
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 was asking Mr Case for answers “in the absence of an independent adviser on ministers’ interests following the resignation of Lord Geidt” in June.
Boris Johnson did not replace him and Ms Truss has indicated that she was not interested in appointing an ethics adviser – saying she knew “the difference between right and wrong”.
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.”
The Labour deputy said: “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.”
Ms Rayner added: “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 Truss has given her “full support” to Mr Fullbrook. A Downing Street spokeswoman said the PM stands 100 per cent 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.”
A spokesman for Mr Fullbrook said he had “fully, completely and voluntarily engaged with the US authorities in this matter”.
The spokesperson added: “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.”
