The US Senate Intelligence Committee has said there are "no indications" Trump Tower was under surveillance by the US government before or after the Presidential election.
President Donald Trump had used a series of Twitter posts to accuse his predecessor, Barack Obama, of wiretapping him.
Committee chairman Richard Burr, and vice chairman Mark Warner, said in a statement: "Based on the information available to us, we see no indications that Trump Tower was the subject of surveillance by any element of the United States government either before or after Election Day 2016."
In an interview on Wednesday with Fox News, Mr Trump suggested he first thought the former president was carrying out surveillance on Trump Tower after reading an article on the campaign in the New York Times and watching a report on the conservative network.
He added: "I watched your friend, Bret Baier, the day previous where he was talking about certain very complex sets of things happening, and wiretapping.
"I said, wait a minute, there’s a lot of wiretapping being talked about. I’ve been seeing a lot of things."
Asked why he did not report his suspicions to the intelligence services before making them public he said he did not want "to do anything that’s going to violate any strength of an agency".
Mr Obama's team has strongly denied Mr Trump's unsubstantiated claims.
Speaking on Thursday, after the Senate committee's statement, White House press secretary Sean Spicer told reporters Mr Trump "stands by" his original allegation.
He added of the committee statement: "They're not findings. There's a statement out today. The statement clearly says that at this time they don't believe that. They've yet to go through the information."
On Wednesday Devin Nunes, the Republican chairman of the House Intelligence Committee, told reporters: "We don’t have any evidence that [wiretapping] took place."
And Attorney General Jeff Sessions said he had not led the President to believe he was under surveillance.
Defending the President, White House press secretary Sean Spicer said earlier this week: "He doesn’t really think that President Obama went up and tapped his phone personally.
"But there’s no question that the Obama administration, that there were actions about surveillance and other activities that occurred in the 2016 election.
"That is a widely reported activity that occurred back then."