FBI Director James Comey is expected to publicly reject Donald Trump’s allegations about wiretapping in a hearing before the House Intelligence Committee.
The White House has refused to drop an accusation made by Mr Trump on Twitter that Barack Obama ordered the wiretapping of Trump Tower during the election campaign despite providing no evidence to back up the campaign.
The claim has caused widespread controversy and the committee launched an investigation at the President’s request, but has yet to find any evidence.
Congressman Adam Schiff, the most high-ranking Democrat on the committee, called Mr Trump’s claims “patently false” and said he expected Mr Comey to agree in his testimony.
His appearance will be the first time Mr Comey has commented publicly on the wiretapping claims despite reports earlier this month that he was urging the Department of Justice to publicly refute them because they imply the FBI acted illegally.
Mr Comey is the only Obama-era national security official to serve in the Trump administration and was widely criticised for announcing a renewed investigation in the use of Hillary Clinton’s private email a few days before November;s presidential election.
Previously unseen emails sent from Ms Clinton to her aide Huma Abedin came to light following an investigation into Ns Abdein's husband - former Congressman Anthony Weiner.
Many top US legislators on both sides of the political divide have denounced the allegations – which have also provoked the ire of the UK when one of its security agencies, GCHQ, was falsely implicated.
"I don't know the basis for President Trump's assertion," US Senator Susan Collins, a Republican, said on NBC's Meet the Press "I do believe he owes us that explanation."
Ms Collins said she supported Mr Trump as president, but she wouldn't side with him if he "misstated what the facts are."
Meanwhile Mr Trump’s approval ratings have fallen to record lows with a Gallup poll showing he now has a 37 per cent approval rate after just two months in the Oval Office – by comparison Mr Obama had an approval rating of 63 per cent in March 2009.
The hearing is also set to examine claims that Moscow interfered in the presidential election and the investigations by several different security agencies.
Mr Comey and NSA Director Admiral Mike Rogers, who is also testifying, are not expected to reveal much in public about the probes, which include information that is classified Top Secret and also separated into different compartments, each of which requires a separate clearance.
But the hearing could become heated as Republicans balance support for their party's leaders and Democrats vent frustration over Republican congressional leaders' refusal to appoint a special prosecutor or select committee to investigate.
Mr Trump fired his first national security adviser, Michael Flynn, last month after he failed to disclose contacts with Russia's ambassador before Mr Trump took office on 20 January.
Attorney General Jeff Sessions, a former senator, recused himself from investigating the matter after it was revealed that he did not answer accurately when he was asked during his confirmation hearing about his contacts with Russian officials during the election.
He failed to disclose that, as senator, he had met with Russia's ambassador to the United States, Sergey Kislyak.
Additional reporting by Reuters