FBI director James Comey, who is Republican, has rejected Donald Trump’s claim that his predecessor, Barack Obama, ordered a wiretap of his phone before he was elected US president, US media say.
Mr Comey reportedly asked the US justice department to publicly reject Saturday’s allegation, according to the New York Times and NBC.
He is said to have asked for the correction because it falsely insinuates that the FBI broke the law.
The department has not commented.
US media quoted officials as saying that Mr Comey believed there was no evidence to support Mr Trump’s allegation.
From an FBI director this is a startling rebuke of a sitting president and he will be under pressure from Democrats to voice it publicly, the BBC’s Nick Bryant reports from Washington.