“WOW! It is now just a question of time before the truth comes out, and when it does, it will be a beauty!” Trump wrote.
The spying claim was roundly denied by U.S. and British intelligence officials when it surfaced two years ago.
At that time, Fox News commentator Andrew Napolitano raised the prospect of British spying on air, and Johnson later acknowledged being a source for Napolitano.
Then-White House press secretary Sean Spicer angered the British government by mentioning the claim during a news briefing. The White House subsequently backed away from the claim, and Fox News disavowed it.
A spokesman for Prime Minister Theresa May said at the time that her government had made clear to U.S. authorities that Johnson’s claims were “ridiculous and should have been ignored.”
In a new statement Wednesday, Britain’s main intelligence agency, Government Communications Headquarters, again dismissed the claim.
“As we have previously stated, the allegations that GCHQ was asked to conduct ‘wire tapping’ against the then President Elect are nonsense,” the statement said.
During his planned visit to Britain in early June, Trump is scheduled to meet with May as well as the queen.
In a statement Tuesday, the White House said the trip would “reaffirm the steadfast and special relationship between the United States and the United Kingdom.”
Johnson, a longtime critic of U.S. intelligence, has been at the center of several other controversies.
In 2008, he spread a false claim that Michelle Obama had been videotaped using a slur against white people.
In 2013, Johnson falsely accused then-Secretary of State John F. Kerry of having committed war crimes in Vietnam, alleging that Kerry had “raped some poor Vietnamese woman.”
More recently, he has maintained that the CIA rather than Russia might have been behind the 2016 hacking of the Democratic National Committee.