Charlotte Hogg's "professional competence falls short" of the standards required to fulfil her role as Bank of England deputy governor, the Treasury Select Committee has said.
In a damning report, the influential committee, headed by Tory MP Andrew Tyrie, stopped short of recommending she be axed from her role.
But it said newly appointed Ms Hogg failed "over a period of nearly four years to comply with the Code of Conduct, despite numerous procedural reminders and opportunities to do so".
The statement comes weeks after Ms Hogg apologised to the committee for not formally disclosing that her brother, Quintin, was Barclays' group strategy director, which could conflict with her work on the Prudential Regulation Committee (PRC).
Ms Hogg had not declared the family link on a number of occasions since joining the Bank as chief operating officer in 2013.
It comes after Ms Hogg, who has been touted as a possible successor to Bank governor Mark Carney, told the Treasury Committee at a hearing that she always declared areas of conflicts of interests and was compliant with all of the Bank's codes of conduct because she helped to write them.
It prompted committee member John Mann to demand she steps down, saying that Ms Hogg "has either lied to Parliament or she is grossly incompetent".