"Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!" Trump wrote.

Just weeks into his new administration, President Donald Trump is mired in a political and diplomatic mess of his own making after leveling unsubstantiated accusations via Twitter that Barack Obama authorized wiretaps on him.



The early morning tweets he sent out from his Mar-a-Lago estate in Palm Beach, Florida on March 4 now threaten to derail the White House's wider agenda and have sparked a spat with close ally Britain.



"Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!" Trump wrote.



"Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A NEW LOW!" he continued, accusing Obama of crimes comparable to those of Richard Nixon's Watergate scandal.



The accusations -- flatly denied by Obama -- were at first seen as frivolous, spur-of-the-moment comments after a politically bruising week, which Trump and his team could later retreat from without much damage.



But two weeks later, the White House has doubled down on the extraordinary accusation against a former president despite calls from his own Republican Party to recant and apologize, and despite a stern rebuke from London.



On Thursday, White House spokesman Sean Spicer, defending his boss, cited a Fox News report that Obama had recruited Britain's signals intelligence agency GCHQ last year to wiretap Trump Tower in New York, a luxury office and residential skyscraper that serves as both Trump's home and offices.



In an uncommonly blunt rebuff, GCHQ branded the Fox claims that Spicer repeated as "nonsense."



"They are utterly ridiculous and should be ignored," a spokesperson said.



After calls from British officials, the White House issued a statement saying Spicer had only mentioned the report and did not mean to suggest it was true.



"Mr Spicer was simply pointing to public reports, not endorsing any specific story," the statement said.



Trump not backing down



Yet the White House still has not backed down from Trump's original claims -- nor has it offered any evidence to support them.



The tweets remain online, and Trump told Fox News late Wednesday that he was going to produce evidence -- eventually.



"Let's see whether or not I prove it. I just don't choose to do it right now," he said. "I think we have some very good stuff. And we're in the process of putting it together, and I think it's going to be very demonstrative."



Yet few in Washington are expecting he can do that.



This week, the top four lawmakers in Congress dealing with intelligence issues, Republicans and Democrats, all said that they have been given no evidence to support Trump's allegations.



Spicer, struggling with intense questioning on the issue at each media briefing, has also not provided any proof.



Federal Bureau of Investigation Director James Comey has so far been silent on the issue, but he is due to testify on Capitol Hill on Monday.



Republicans press for Trump apology



On Friday, Republicans in Congress, having earlier skirted the issue, showed concern that it was overwhelming the need to focus on major policy issues.



Several called on Trump to admit his blunder and apologize to Obama.



"I see no indication that that's true," Representative Tom Cole told CNN. "Frankly, unless you can produce some pretty compelling proof, then I think president Obama is owed an apology."



"It's inexplicable," said Representative Charlie Dent. "I think he should simply retract it."