GCHQ called the claims "utterly ridiculous" even as White House spokesman Sean Spicer repeated President Donald Trump's allegations.
A British spy agency on Thursday denied allegations that it helped the Obama administration "wiretap" US President Donald Trump's phones during the 2016 presidential campaign.
In a rare public statement, the spy agency, GCHQ, termed the claims "utterly ridiculous" that "should be ignored."
"Recent allegations made by media commentator judge Andrew Napolitano about GCHQ being asked to conduct 'wiretapping' against the then president-elect are nonsense," The Guardian quoted a GCHQ spokesperson as saying.
On Thursday White House spokesman Sean Spicer said that Trump stood by his accusations even as three senior lawmakers rejected the claims.
"He [Trump] stands by it," Spicer said.
The White House cited unproven media reports that former US President Barack Obama asked Britain's signals intelligence agency to monitor Trump in order to "make sure there were no American fingerprints."
Spicer quoted at length from a Fox News report, which alleged Obama had used GCHQ to dodge US legal restrictions on monitoring US citizens.
His comments came after the Republican and Democratic leaders of the Senate Intelligence Committee said that they saw no evidence to support Trump's claim.
"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," Republican Chairman Richard Burr and Senator Mark Warner, the committee's Democratic vice chairman, said in a statement.
The top Republican in Congress, House of Representatives Speaker Paul Ryan, on Thursday added his voice to those saying there was no sign of a wiretap.
Trump, a Republican, made the accusation six weeks after he took over the presidency from Democrat Barack Obama.
In a series of tweets on March 4, he accused Obama of a "Nixon/Watergate"-like plot that would almost certainly break US law.
Obama said through a spokesman that it was "simply false."
At least four congressional committees added the startling accusation in their investigations of possible Russian meddling in the election campaign and Russian ties to Trump and his associates.