President Donald Trump's accusation that his predecessor ordered his phones to be tapped is "simply false", Barack Obama's spokesman has said.
Kevin Lewis said that "neither President Obama nor any White House official ever ordered surveillance on any US citizen".
Mr Trump had tweeted: "Terrible! Just found out Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
He gave no details to back the claim.
In his statement, Mr Lewis said a "cardinal rule of the Obama Administration was that no White House official ever interfered with any independent investigation led by the Department of Justice".
The statement left open the possibility that a judicial investigation was taking place.
Earlier Ben Rhodes, who was Mr Obama's foreign policy adviser and speechwriter, also addressed Mr Trump's claims in a tweet, saying: "No President can order a wiretap. Those restrictions were put in place to protect citizens from people like you."
Mr Trump, who is at his Florida resort, fired off a series of tweets from just after 06:30 local time (11:30 GMT) on Saturday.
He called the alleged tapping "a new low" and said "This is Nixon/Watergate" - referring to the most notorious political scandal of 1972, which led to the downfall of President Richard Nixon after a web of political spying, sabotage and bribery was exposed by the media.
McCarthyism, which Mr Trump referred to in one of the first posts, relates to the persecution for US Communists and their allies led by Senator Joe McCarthy in the 1950s.
The tweets followed allegations made by conservative radio host Mark Levin, which were later picked up by Breitbart News, the website run by Steve Bannon before he became Mr Trump's chief strategist.
Mr Levin said there should be a congressional investigation into what he called President Obama's "police state" tactics in his last months in office to undermine Mr Trump's campaign.
Breitbart summarises Mr Levin's accusations, which say that "the Obama administration sought, and eventually obtained, authorisation to eavesdrop on the Trump campaign; continued monitoring the Trump team even when no evidence of wrongdoing was found; then relaxed the NSA (National Security Agency) rules to allow evidence to be shared widely within the government".
This piece originally appeared at the BBC.