The US intelligence service watchdog has shot down a conspiracy theory promoted by Donald Trump in his attacks on the credibility of the Ukraine whistleblower.

In a rare public intervention, the Office of the Inspector General of the Intelligence Community rejected suggestions that a complaint that has left the president facing an impeachment probe was based on second-hand information.

It also debunked false claims – also made by other Republicans – that whistleblowing rules were loosened shortly before the complaint was filed to remove a requirement of direct evidence of wrongdoing.

The inspector general’s statement was issued on Monday after Mr Trump wrote on Twitter: “WHO CHANGED THE LONG STANDING WHISTLEBLOWER RULES JUST BEFORE SUBMITTAL OF THE FAKE WHISTLEBLOWER REPORT?”

The president’s tweet appeared to refer to a conspiracy theory first propagated by an article on right-wing website The Federalist. The piece claimed the intelligence community last month “secretly eliminated a requirement that whistleblowers provide direct, first-hand knowledge of alleged wrongdoings”

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The article suggested this “raises questions about the intelligence community’s behaviour” over the whistleblower’s complaint about a phone call in which Mr Trump pressured Ukraine’s president to investigate unfounded corruption allegations against his Democratic rival Joe Biden.

The story was widely shared by Republican members of congress and conservative commentators over the weekend. But the inspector general’s intervention makes it clear the article is incorrect on multiple points.

The office said the form used by the whistleblower to submit the complaint on 12 August this year had been in use since 24 May 2018. First-hand knowledge of an event has never been required for a complaint, it added.

“Although the form requests information about whether the Complainant possesses first-hand knowledge about the matter about which he or she is lodging the complaint, there is no such requirement set forth in the statute,” said the statement.

It added that the whistleblower had, in any case, “stated on the form that he or she possessed both first-hand and other information”.

“The ICIG reviewed the information provided as well as other information gathered and determined that the complaint was both urgent and that it appeared credible,” the statement read.

The office said Michael Atkinson, the inspector general, determined during a preliminary review that the whistleblower had “direct knowledge of certain alleged conduct”.

Other information obtained during the review supported the allegations, which led Democrats to announce a formal impeachment inquiry into Mr Trump last week.

The whistleblower’s report claimed the president had withheld nearly $400m in military aid from Ukraine as he pressured the eastern European nation to investigate Mr Biden and his son Hunter.