The president is under increasing pressure to justify his claims, which his opponents charge calls the whole integrity of his administration into question.
In an attempt to provide credibility to the claims, Mr Spicer quoted from a series of articles which discussed surveillance.
He referenced comments made earlier this week on Fox News TV by Mr Napolitano in relation to Mr Trump’s controversial claim that wiretaps had been installed at his New York residence:
Three intelligence sources have informed Fox News that President Obama went outside the chain of command. He didn’t use the NSA, he didn’t use the CIA, he didn’t use the FBI and he didn’t use the Department of Justice.
He used GCHQ. What is that? It’s the initials for the British intelligence-finding agency. So, simply by having two people saying to them president needs transcripts of conversations involving candidate Trump’s conversations, involving president-elect Trump, he’s able to get it and there’s no American fingerprints on this. Putting the published accounts and common-sense together, this leads to a lot.
British officials were quick to rubbish Mr Napolitano’s claims. A government source reportedly said the claim was “totally untrue and quite frankly absurd”.
The British official told Reuters that under British law, GCHQ “can only gather intelligence for national security purposes” and noted that a US election “clearly doesn’t meet that criteria”.
Mr Spicer’s press conference on Thursday was held shortly after the senate intelligence committee published a statement saying they had no evidence for Mr Trump’s claim, made on March 4, that Mr Obama ordered wiretaps on Trump Tower.