Donald Trump’s claim that has he was wiretapped by Barack Obama is under increasing assault after a congressional committee said it had seen no evidence to support the allegation and it did not believe that it happened.

The president startled millions earlier this month when he claimed – without providing any evidence – that his predecessor at the White House had ordered electronic surveillance of Trump Tower during the election campaign.

The White House asked congress to investigate the claim, and the two committees that agreed to do so asked the Department of Justice for information to support the accusation. On Tuesday, the White House said Mr Trump was “very confident” he would be vindicated.

But on Wednesday, Devin Nunes, the Republican Chairman of the House Intelligence Committee, said he did not believe Mr Trump’s phones had been tapped.

“As I told you last week, in regard to the president talking about tapping Trump Tower. That evidence still remains the same. We don’t have any evidence that took place,” he said.

“And I don’t believe, having spoken to people just in the last week, I don’t believe there was an actual tap of Trump Tower.”

He added: “Now you have to decide if you are going to take the tweets literally? If you are, clearly the president is wrong.”

Jeff Sessions hesitates, then denies he ever gave a reason for Trump to believe he was wiretapped

Congressman Adam Schiff, the senior Democrat on the committee, said: “It deeply concerns me that the president would make such an accusation without basis.”

Mr Trump’s spokesman, Sean Spicer, has sought to walk back the president’s claim, made in a series of early morning tweets, and suggested he was talking about broader surveillance, and did not necessarily mean wiretapping.

“I will let the committees do their jobs, but there is significant reporting about surveillance techniques during the 2016 election,” he said this week.

“I will leave it up to the [congressional committees] to issue their reports. But he feels very confident that ultimately [he will be] vindicated.”

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Mr Nunes said that while he did not believe Mr Trump had been wiretapped, he said he remained concerned about two issues – whether Americans had been electronically eavesdropped on and if anyone had been “unmasked’ by the intelligence services. He said there was evidence that crimes were committed in revealing communications between former National Security Advisor Michael Flynn and the Russian ambassador to the US.

“For a long time here I have been very clear about my concern about, number one, the incidental collection on General Flynn, how that was put into a product, how it was unmasked, how it was leaked to the public,” he said. “Several crimes have been committed here.”

Republican senator Lindsey Graham said Congress was ready to “flex its muscles” if FBI Director James Comey did not answer questions by Wednesday about any Russian ties to the Trump campaign or provide evidence of a wiretapping warrant for Trump Tower.

Mr Graham, in a series of televised interviews, said he was prepared to issue subpoenas for information on whether the FBI received a warrant to tap Mr Trump's phones during the presidential campaign.