A cross-part committee was furious that Mr Sharp failed to declare to MPs his role in facilitating the loan when he was applying for the job of BBC chairman and said he should "consider the impact his omissions will have" on trust in the broadcaster.
They said his actions "constitute a breach of the standards expected of individuals" applying for prominent public appointments.
Mr Sharp has insisted that he did not arrange the loan, but admitted introducing his friend Sam Blyth, a cousin of Mr Johnson who wanted to help the then Prime Minister with his financial troubles, to the Cabinet Office.
A spokesman for Mr Sharp said he "regrets" not telling MPs about his involvement with Mr Blyth "and apologises".
Mr Sharp was named as the preferred candidate for the BBC job in January 2021 and the Commons Digital, Culture, Media and Sport Committee backed his appointment - but crucially they were not aware of his role in facilitating the £800,000 loan guarantee.
In a strongly-worded report they have now suggested Mr Sharp's failure to come clean could damage the BBC.
"Richard Sharp's decisions, firstly to become involved in the facilitation of a loan to the then-prime minister while at the same time applying for a job that was in that same person's gift, and then to fail to disclose this material relationship, were significant errors of judgment, which undermine confidence in the public appointments process and could deter qualified individuals from applying for such posts," the MPs said.
The committee concluded: "Mr Sharp should consider the impact his omissions will have on trust in him, the BBC and the public appointments process."
The MPs were also critical of Rishi Sunak and other senior ministers who had highlighted their 2021 decision to endorse Mr Sharp to defend the appointment since the row over the loan broke, despite the fact they had not been told about the situation.
"The fact that ministers have cited this committee's original report on Mr Sharp's appointment as a defence of the process was followed, when we were not in full possession of all the facts that we should have had before us in order to come to our judgment, is highly unsatisfactory," the MPs said.
The MPs said there was an "unresolved issue" as to why Cabinet Secretary Simon Case believed Mr Sharp had himself been giving financial advice to Mr Johnson and called on the Cabinet Office to "clear up the confusion".
A spokesman for Mr Sharp said the BBC chairman "appreciates that there was information that the committee felt that it should have been made aware of in his pre-appointment hearing".
"He regrets this and apologises," the spokesman said. "It was in seeking at the time to ensure that the rules were followed, and in the belief that this had been achieved, that Mr Sharp acted in good faith.
"Mr Sharp believed he had dealt with the issue by proactively briefing the Cabinet Secretary that he was applying for the role of BBC chair.