The White House knew National Security Adviser Michael Flynn was vulnerable to blackmail by the Russian government at least 18 days before he was forced out of his job, according to the former acting US Attorney General Sally Yates who has testified to a Senate committee that she warned officials he was “compromised”.

Mr Flynn was forced to resign from President Donald Trump's administration having seemingly misled the White House over contact he had with Russian ambassador to Washington, Sergey Kislyak, in a scandal that rocked the early days of Mr Trump's presidency. The hearing was clearly on Mr Trump's mind, with the President claiming in a tweet following the testimony that it was “nothing but old news”.

Ms Yates testified that she met with White House Counsel Donald McGahn on 26 January to tell him that she had information that public statements by Vice President Mike Pence about the brief nature of conversations between Mr Flynn and Mr Kislyak were false – and that the conversations in December were extensive and included discussions about US sanctions on Russia.

“The Russians also knew about what General Flynn had done, and the Russians also knew that General Flynn had misled the Vice President and others,” Ms Yates said before the Senate Judiciary subcommittee. “That created ... A situation where the national security adviser essentially could be blackmailed by the Russians.”

“To state the obvious: You don’t want your national security adviser compromised with the Russians,” Ms Yates added.

Speaking for the first time publicly about the first big scandal of the Trump presidency, Ms Yate's testimony is part of the larger investigations by both sides of Congress into alleged Russian interference in the 2016 presidential election, with US intelligence services believing this was aimed at helping Mr Trump. The House and the Senate are also investigating whether there was any collusion between members of Mr Trump's team and Russia.

Ms Yates was appearing in front of the committee with James Clapper, Director of National Intelligence under President Barack Obama who retired when Mr Trump took office – but it was clear Ms Yates was the box office draw.

The controversial orders Donald Trump has already issued Show all 9 1 /9 The controversial orders Donald Trump has already issued The controversial orders Donald Trump has already issued Trump and the media White House Press Secretary Sean Spicer takes questions during the daily press briefing Getty Images The controversial orders Donald Trump has already issued Trump and the Trans-Pacific Partnership Union leaders applaud US President Donald Trump for signing an executive order withdrawing the US from the Trans-Pacific Partnership negotiations during a meeting in the Roosevelt Room of the White House in Washington DC. Mr Trump issued a presidential memorandum in January announcing that the US would withdraw from the trade deal Getty The controversial orders Donald Trump has already issued Trump and the Mexico wall A US Border Patrol vehicle sits waiting for illegal immigrants at a fence opening near the US-Mexico border near McAllen, Texas. The number of incoming immigrants has surged ahead of the upcoming Presidential inauguration of Donald Trump, who has pledged to build a wall along the US-Mexico border. A signature campaign promise, Mr Trump outlined his intention to build a border wall on the US-Mexico border days after taking office Getty Images The controversial orders Donald Trump has already issued Trump and abortion US President Donald Trump signs an executive order as Chief of Staff Reince Priebus looks on in the Oval Office of the White House. Mr Trump reinstated a ban on American financial aide being granted to non-governmental organizations that provide abortion counseling, provide abortion referrals, or advocate for abortion access outside of the United States Getty Images The controversial orders Donald Trump has already issued Trump and the Dakota Access pipeline Opponents of the Keystone XL and Dakota Access pipelines hold a rally as they protest US President Donald Trump's executive orders advancing their construction, at Columbus Circle in New York. US President Donald Trump signed executive orders reviving the construction of two controversial oil pipelines, but said the projects would be subject to renegotiation Getty Images The controversial orders Donald Trump has already issued Trump and 'Obamacare' Nancy Pelosi who is the minority leader of the House of Representatives speaks beside House Democrats at an event to protect the Affordable Care Act in Los Angeles, California. US President Donald Trump's effort to make good on his campaign promise to repeal and replace the healthcare law failed when Republicans failed to get enough votes. Mr Trump has promised to revisit the matter Getty Images The controversial orders Donald Trump has already issued Donald Trump and 'sanctuary cities' US President Donald Trump signed an executive order in January threatening to pull funding for so-called "sanctuary cities" if they do not comply with federal immigration law AP The controversial orders Donald Trump has already issued Trump and the travel ban US President Donald Trump has attempted twice to restrict travel into the United States from several predominantly Muslim countries. The first attempt, in February, was met with swift opposition from protesters who flocked to airports around the country. That travel ban was later blocked by the Ninth Circuit Court of Appeals. The second ban was blocked by a federal judge a day before it was scheduled to be implemented in mid-March SANDY HUFFAKER/AFP/Getty Images The controversial orders Donald Trump has already issued Trump and climate change US President Donald Trump sought to dismantle several of his predecessor's actions on climate change in March. His order instructed the Environmental Protection Agency to reevaluate the Clean Power Plan, which would cap power plant emissions Shannon Stapleton/Reuters

During her testimony, Ms Yates said she again met with Mr McGahn on 27 January to discuss his concerns. She said Mr McGahn had asked her how Mr Flynn had fared during an interview with the FBI earlier that week, as well as why it would matter to the Justice Department if one White House official lies to another. He also asked about the likelihood that the agency would pursue a criminal case.

Mr McGahn requested to see the underlying evidence, Ms Yates said, but added that she did not know if the Justice Department followed up with this request.

Ms Yates was fired as acting Attorney General by Mr Trump over the unrelated matter of her refusal to defend his first attempt at a travel ban, which would have restricted travel from seven majority-Muslim countries. Mr Flynn resigned from his post on 13 February, after media reports began to surface about his actions.

Obama takes dig at Trump during award acceptance speech: 'We need courage to stand up to hate'

Democrats on the committee expressed concern as to why Mr Flynn wasn't immediately dismissed. Senator Al Franken said that he didn’t understand why Mr McGahn didn’t realise why it matters to the Justice Department if White House officials lie to each other.

Mr Franken said that there are policies that deal with who receives security clearance and who does not, and that clearance holders – including Mr Flynn – must always demonstrate trustworthiness and freedom from potential coercion.

“Why wouldn’t you a fire a guy who did this?” Mr Franken said, referring to why Mr Trump did not dismiss Mr Flynn for his misleading statements immediately. Mr Franken speculated that the only explanation he could think of was that there were other Trump administration officials who had contacts with the Russian government.

It was a day of drama outside the hearing too, with news that Mr Obama had warned Mr Trump, then president-elect, not to give the post of National Security Adviser in his administration to Mr Flynn.

The Democratic president gave the warning in an Oval Office meeting with Mr Trump two days after the Republican's surprise election win on 8 November. The warning, first reported by NBC News via former Obama aides, came up during a discussion of White House personnel.

White House spokesman Sean Spicer, confirmed that was the case: “It's true that the president, President Obama, made it known that he wasn't exactly a fan of General Flynn's, which frankly shouldn't come as a surprise to anyone, given that General Flynn had worked for President Obama, was an outspoken critic of President Obama's shortcomings.”

Mr Flynn had been pushed out by Mr Obama in 2014 from his job as director of the military's Defense Intelligence Agency, or DIA – which he had taken up in 2012.

The near three-hour hearing often veered off topic. Republican Senators repeatedly pressed Ms Yates on her refusal to defend the Trump administration's travel ban.

The day ended as it began with Mr Trump sounding off about the hearing on Twitter. Hours before the testimony, Mr Trump insinuated that Ms Yates may have leaked information on Mr Flynn to the media.

“Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Council,” Mr Trump wrote on Twitter, apparently misspelling the word counsel. Ms Yates said during the hearing she did not leak classified information. Mr Trump also tweeted that it was the Obama administration, not he, that had given Mr Flynn “the highest security clearance” when he worked at the Pentagon.