Calls are getting louder on both sides of the Atlantic for U.S. President Donald Trump to retract and apologize for saying former president Barack Obama, with the help of Britain's spy agency, wiretapped his New York skyscraper.

No evidence has been found to back up the claims, which have drawn bipartisan criticism in the U.S. and scorn in the U.K.

'President Obama is owed an apology.' - Tom Cole, Republican congressman

On Friday, the White House promised the allegations against U.K. spies won't be repeated, according to a spokesperson for British Prime Minister Theresa May.

James Slack said the "ridiculous" claims should be ignored. 

Republican congressman Tom Cole, of Oklahoma, said Friday that Trump should apologize to Obama for the unproven claim.  

"I think the president, president Obama, is owed an apology in that regard, because if he didn't do it, we shouldn't be reckless in accusations that he did."​

Another Republican congressman, Charlie Dent of Pennsylvania, said there is no proof of Trump's claim and "he should simply retract it and move on."

White House spokesperson Sean Spicer angrily defended the president's statement, telling reporters on Thursday that Trump "stands by" the four tweets that sparked a firestorm. Spicer cited an unsubstantiated report that Britain's spy agency, GCHQ, helped Obama eavesdrop on Trump during the election campaign.

Spicer also denounced reporters for taking the president's words too literally. 

Spicer's comments were a rebuttal to the Senate intelligence committee's top two members, who released a statement earlier Thursday declaring there is no indication that Trump Tower was "the subject of surveillance" by the U.S. government before or after the 2016 election.

Spicer suggested the statement from senators Richard Burr, a Republican from North Carolina, and Mark Warner, a Virginia Democrat, was made without a full review of the evidence or, incorrectly, a briefing from the Justice Department.

"They are not findings," he said.

Trump Russia

Security personnel stand at the front entrance of Trump Tower last year. The U.S. president on Twitter accused former president Barack Obama of tapping the phones at his New York skyscraper. (Seth Wenig/Associated Press)

The standoff between the White House and lawmakers came four days before FBI Director James Comey is slated to testify before Congress, when he will inevitably be asked whether the president's accusations are accurate. The White House's refusal to back down raised the stakes for Comey's appearance before the intelligence committee on Monday.

Trump tweeted earlier this month that Obama "was tapping my phones in October," and compared the incident to "Nixon/Watergate" and "McCarthyism."

Trump, in an interview Wednesday with Fox News, said there would be "some very interesting items coming to the forefront over the next two weeks."

Britain's GCHQ

Spicer on Thursday asserted that Trump meant to broadly refer to "surveillance," rather than a phone wiretap.

In an attempt to bolster his case, the spokesman spent nearly 10 minutes angrily reading from news reports which he said pointed to possible evidence of surveillance. He also quoted a transcript of an appearance by Fox News analyst Andrew Napolitano on the network, when he suggested GCHQ had helped with the alleged tapping. Obama, he claimed, "went outside the chain of command" so there were "no American fingerprints on this."

Trump Russia

Senate intelligence committee chairman Richard Burr, right, and committee vice-chairman Mark Warner said in a one-sentence joint statement Thursday there is no indication that Trump Tower was 'the subject of surveillance' by the U.S. government before or after the 2016 election. (Alex Brandon/Associated Press)

The agency, which rarely comments on allegations about intelligence matters, flatly denied the claim, responding with a statement calling the allegations "nonsense."

"They are utterly ridiculous and should be ignored," read the statement, which was issued on condition that it be attributed to an anonymous spokesperson to protect the identity of agency staff.

GCHQ has a close relationship with the NSA, as well as with the eavesdropping agencies of Australia, Canada and New Zealand in a consortium called Five Eyes.

A British official told Reuters that under British law, GCHQ "can only gather intelligence for national security purposes" and noted that the U.S. election "clearly doesn't meet that criteria."

The official added that GCHQ "can only carry out intelligence operations where it is legal in both the U.S. and UK to do so."

Under U.S. law, presidents cannot direct wiretapping. Instead, the federal government can ask a court to authorize the action, but it must provide justification.

Burr and Warner were among eight senior congressional leaders briefed by Comey on March 10. A Senate aide, who requested anonymity to discuss the senators' private briefings, said Spicer was incorrect in claiming Burr and Warner had not been briefed on the matter.

"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," Burr and Warner said in a one-sentence joint statement Thursday afternoon.

With files from CBC News