The federal probe into possible collusion between Donald Trump's campaign team and Russia has now struck at the heart of the White House, with Donald Trump’s former national security adviser admitting that he lied to the FBI about contacts with a senior Russian diplomat.

In a crucial breakthrough for prosecutors, Michael Flynn appeared in a Washington DC courtroom to plead guilty to one count of making false statements to investigators and announced that he was fully cooperating with Special Counsel Robert Mueller's team.

Mr Flynn is the first person within Mr Trump's administration to be charged by the probe in a milestone for investigators, and the details in his plea agreement have turned attention back to officials in the president's inner-circle. Mr Flynn said that in December 2016 - as Mr Trump was preparing to take office - he was instructed to contact Russia’s Washington Ambassador at the time, Sergey Kislyak, taking direction from a "very senior member" of the transition team. He also said that he had discussed what to say to Mr Kislyak with other "senior members" of the team.

While the official was not named in court documents, multiple reports suggest that Jared Kushner, a senior White House adviser and Mr Trump's son-in-law, could be the "very senior member" involved in conversations.

Mr Flynn, a former army general, admitted that during telephone conversations with Mr Kislyak, he asked for Russia to try to delay a UN resolution criticising Israeli settlements and not to respond in kind to the expulsion of 35 Russian diplomats by Barack Obama in response to Moscow’s alleged interference in the 2016 election. A day after the second request to Mr Kislyak, Vladimir Putin announced he was not going to expel US diplomats, a move that was subsequently praised by Mr Trump.

Mr Flynn, 58, appeared in court in Washington. He cut a businesslike figure in a dark suit, offering only perfunctory answers to questions from Judge Rudolph Contreras. Mr Flynn acknowledged the government’s summary of the charge to which he was pleading guilty was correct. He also said that he understood all possible consequences of his plea agreement and that his sentence could end up being either more, or less than the sentencing guidelines permit. The charge brings a potential sentence of up to five years in prison.

After the hearing Mr Flynn issued a statement in which he spoke of the pain of enduring allegations of treason - accusations he denied.

Yet he added: “I recognise that the actions I acknowledged in court today were wrong, and, through my faith in God, I am working to set things right. My guilty plea and agreement to cooperate with the Special Counsel’s office reflect a decision I made in the best interests of my family and of our country. I accept full responsibility for my actions.”

Michael Flynn once said anyone seeking immunity 'probably committed a crime'

President Trump's lawyers also moved quickly to assign blame to the former national security adviser. They said he had been fired as soon as it became clear that he had lied about his contacts with Mr Kislyak.

“Nothing about the guilty plea or the charge implicates anyone other than Mr Flynn,” said Ty Cobb, the President’s lawyer leading the team dealing with the Russia inquiry.

Mr Flynn pleaded guilty to one charge of lying to investigators when he appeared in court (AP)

Court documents show that Mr Flynn told investigators that on December 29, he called "a senior transition official", who was with other members of the team at Mr Trump’s Mar-a-Lago club in Florida “to discuss what, if anything, to communicate to the Russian ambassador about the US sanctions”.

The documents say that on December 22 2016, a "very senior member" of the team contacted him and asked him to speak with the Russian envoy about the UN vote on Israel as well as a number of other countries. Mr Kushner has been handed the task of helping to achieve a peace deal between the Israelis and the Palestinians.

It is understood Mr Kushner was in Hawaii on December 29 2016. It is believed he was one of several senior officials within the Trump transition team working to avoid the UN criticism of Israel.

Senator Mark Warner, the senior Democrat on the Senate Intelligence Committee, said he would wish to interview Mr Kushner again, with Mr Kushner having previously testified in camera.

“There are a number, like Mr Kushner and others, that we’re going to want to invite back,” said Mr Warner told reporters. He declined to say whether that would include Vice President Mike Pence, who was also among the senior figures within Mr Trump’s transition team.

The biggest names involved in the Trump-Russia investigation Show all 17 1 /17 The biggest names involved in the Trump-Russia investigation The biggest names involved in the Trump-Russia investigation Paul Manafort Mr Manafort is a Republican strategist and former Trump campaign manager. He resigned from that post over questions about his extensive lobbying overseas, including in Ukraine where he represented pro-Russian interests. Mr Manafort turned himself in at FBI headquarters to special counsel Robert Mueller’s team on Oct 30, 2017, after he was indicted under seal on charges that include conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading US Foreign Agents Registration Act statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts. Getty The biggest names involved in the Trump-Russia investigation Rick Gates Mr Gates joined the Trump team in spring 2016, and served as a top aide until he left to work at the Republican National Committee after the departure of former Trump campaign manager Paul Manafort. Mr Gates' had previously worked on several presidential campaigns, on international political campaigns in Europe and Africa, and had 15 years of political or financial experience with multinational firms, according to his bio. Mr Gates was indicted alongside Mr Manafort by special counsel Robert Mueller's team on charges that include conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading US Foreign Agents Registration Act statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts. AP The biggest names involved in the Trump-Russia investigation George Papadopoulos George Papadopoulos was a former foreign policy adviser for the Trump campaign, having joined around March 2016. Mr Papadopoulos plead guilty to federal charges for lying to the FBI as a part of a cooperation agreement with Special Counsel Robert Mueller's investigation. Mr Papadopoulos claimed in an interview with the FBI that he had made contacts with Russian sources before joining the Trump campaign, but he actually began working with them after joining the team. Mr Papadopoulos allegedly took a meeting with a professor in London who reportedly told him that Russians had "dirt" on Hillary Clinton. The professor also allegedly introduced Mr Papadopoulos to a Russian who was said to have close ties to officials at the Russian Ministry of Foreign Affairs. Mr Papadopoulos also allegedly was in contact with a woman whom he incorrectly described in one email to others in the campaign as the "niece" to Russian President Vladimir Putin. Twitter The biggest names involved in the Trump-Russia investigation Donald Trump Jr The President's eldest son met with a Russian lawyer - Natalia Veselnitskaya - on 9 June 2016 at Trump Tower in New York. He said in an initial statement that the meeting was about Russia halting adoptions of its children by US citizens. Then, he said it was regarding the Magnitsky Act, a US law blacklisting Russian human rights abusers. In a final statement, Mr Trump Jr released a chain of emails that revealed he took the meeting in hopes of getting information Ms Veselnitskaya had about Hillary Clinton's alleged financial ties to Russia. He and the President called it standard "opposition research" in the course of campaigning and that no information came from the meeting. The meeting was set up by an intermediary, Rob Goldstone. Jared Kushner and Paul Manafort were also at the same meeting. Getty Images The biggest names involved in the Trump-Russia investigation Jared Kushner Mr Kushner is President Donald Trump's son-in-law and a key adviser to the White House. He met with a Russian banker appointed by Russian President Vladimir Putin in December. Mr Kushner has said he did so in his role as an adviser to Mr Trump while the bank says he did so as a private developer. Mr Kushner has also volunteered to testify in the Senate about his role helping to arrange meetings between Trump advisers and Russian Ambassador to the US Sergey Kislyak. Getty Images The biggest names involved in the Trump-Russia investigation Rob Goldstone Former tabloid journalist and now music publicist Rob Goldstone is a contact of the Trump family through the previously Trump-owned 2013 Miss Universe pageant, which took place in Moscow. In June 2016, he wrote to Donald Trump Jr offering a meeting with a Russian lawyer, Natalya Veselnitskaya, who had information about Hillary Clinton. Mr Goldstone was the intermediary for Russian pop star Emin Agalaraov and his father, real estate magnate Aras, who played a role in putting on the 2013 pageant. In an email chain released by Mr Trump Jr, Mr Goldstone seemed to indicate Russian government's support of Donald Trump's campaign. AP images The biggest names involved in the Trump-Russia investigation Aras and Emin Agalarov Aras Agalarov (R) is a wealthy Moscow-based real estate magnate and son Emin (L) is a pop star. Both played a role in putting on the previously Trump-owned 2013 Miss Universe pageant in Moscow. They allegedly had information about Hillary Clinton and offered that information to the Trump campaign through a lawyer with whom they had worked with, Natalia Veselnitskaya, and music publicist Rob Goldstone. Getty Images The biggest names involved in the Trump-Russia investigation Natalia Veselnitskaya Natalia Veselnitskaya is a Russian lawyer with ties to the Kremlin. She has worked on real estate issues and reportedly counted the FSB as a client in the past. She has ties to a Trump family connection, real estate magnate Aras Agalarov, who had helped set up the Trump-owned 2013 Miss Universe pageant which took place in Moscow. Ms Veselnitskaya met with Donald Trump Jr, Jared Kushner, and Paul Manafort in Trump Tower on 9 June 2016 but denies the allegation that she went there promising information on Hillary Clinton's alleged financial ties to Russia. She contends that the meeting was about the US adoptions of Russian children being stopped by Moscow as a reaction to the Magnitsky Act, a US law blacklisting Russian human rights abusers. Getty Images The biggest names involved in the Trump-Russia investigation Mike Flynn Mr Flynn was named as Trump's national security adviser but was forced to resign from his post for inappropriate communication with Russian Ambassador to the US Sergey Kislyak. He had misrepresented a conversation he had with Mr Kislyak to Vice President Mike Pence, telling him wrongly that he had not discussed sanctions with the Russian. Getty Images The biggest names involved in the Trump-Russia investigation Sergey Kislyak Mr Kislyak, the former longtime Russian ambassador to the US, is at the centre of the web said to connect President Donald Trump's campaign with Russia. Reuters The biggest names involved in the Trump-Russia investigation Roger Stone Mr Stone is a former Trump adviser who worked on the political campaigns of Richard Nixon, George HW Bush, and Ronald Reagan. Mr Stone claimed repeatedly in the final months of the campaign that he had backchannel communications with WikiLeaks founder Julian Assange and that he knew the group was going to dump damaging documents to the campaign of Democratic candidate Hillary Clinton - which did happen. Mr Stone also had contacts with the hacker Guccier 2.0 on Twitter, who claimed to have hacked the DNC and is linked to Russian intelligence services. Getty Images The biggest names involved in the Trump-Russia investigation Jeff Sessions The US attorney general was forced to recuse himself from the Trump-Russia investigation after it was learned that he had lied about meeting with Russian Ambassador to the US Sergey Kislyak. Getty Images The biggest names involved in the Trump-Russia investigation Carter Page Mr Page is a former advisor to the Trump campaign and has a background working as an investment banker at Merrill Lynch. Mr Page met with Russian Ambassador to the US Sergey Kislyak during the 2016 Republican National Convention in Cleveland. Mr Page had invested in oil companies connected to Russia and had admitted that US Russia sanctions had hurt his bottom line. Reuters The biggest names involved in the Trump-Russia investigation Jeffrey "JD" Gorden Mr Gordon met with Russian Ambassador to the US Sergey Kislyak during the 2016 Republian National Convention to discuss how the US and Russia could work together to combat Islamist extremism should then-Republican presidential candidate Donald Trump win the election. The meeting came days before a massive leak of DNC emails that has been connected to Russia. Creative Commons The biggest names involved in the Trump-Russia investigation James Comey Mr Comey was fired from his post as head of the FBI by President Donald Trump. The timing of Mr Comey's firing raised questions around whether or not the FBI's investigation into the Trump campaign may have played a role in the decision. Getty Images The biggest names involved in the Trump-Russia investigation Preet Bharara Mr Bahara refused, alongside 46 other US district attorney's across the country, to resign once President Donald Trump took office after previous assurances from Mr Trump that he would keep his job. Mr Bahara had been heading up several investigations including one into one of President Donald Trump's favorite cable television channels Fox News. Several investigations would lead back to that district, too, including those into Mr Trump's campaign ties to Russia, and Mr Trump's assertion that Trump Tower was wiretapped on orders from his predecessor. Getty Images The biggest names involved in the Trump-Russia investigation Sally Yates Ms Yates, a former Deputy Attorney General, was running the Justice Department while President Donald Trump's pick for attorney general awaited confirmation. Ms Yates was later fired by Mr Trump from her temporary post over her refusal to implement Mr Trump's first travel ban. She had also warned the White House about potential ties former National Security Adviser Michael Flynn to Russia after discovering those ties during the FBI's investigation into the Trump campaign's connections to Russia. Getty Images

Democrats and other critics of Mr Trump were quick to leap on the developments.

“The guilty plea of President Trump’s former national security adviser to lying to the FBI about his communications with Russian authorities marks a dark moment in our nation’s history,” said Nancy Pelosi, the Democratic leader in the House.

Mr Flynn was the fourth person connected to the Trump campaign to be charged by Special Counsel Robert Mueller. In October, Mr Trump’s former campaign manager, Paul Manafort, and his associate Rick Gates, were charged with 12 counts of financial crimes related to their work in Ukraine. At the same time, Mr Mueller revealed foreign policy adviser, George Papadopoulos, had pleaded guilty to lying to the FBI about contacts with Russia-linked individuals - a similar charge that Mr Flynn admitted to as part of the plea deal.

Former US national security adviser Michael Flynn arrives at court

Mr Trump and his lawyers are likely to be intensely concerned that someone who was a key figure within the campaign, was now cooperating with the federal probe.

“The fact that Flynn was charged with, and is pleading guilty to, such a minor crime, suggests a bombshell of a deal with prosecutors,” said Jens Ohlin, a professor of law at Cornell Law School.

“Flynn was facing serious criminal liability for a variety of alleged missteps, including his failure to register as an agent of a foreign power.”

He added: “If this is the entirety of the plea deal, the best explanation for why Mueller would agree to it is that Flynn has something very valuable to offer in exchange: damaging testimony on someone else. Who? That’s not clear, but it probably is someone at the centre, or close to the centre, of this criminal enterprise.”

Mr Trump has yet to publicly comment on developments. He ignored questions as he welcomed the Libyan prime minister to the White House on Friday afternoon and officials cancelled a press event the two leaders were due to hold.

Both Mr Warner and Congressman Adam Schiff of California, another Democrat, issued statements saying it would be unacceptable for Mr Trump to try to influence any investigations, including the probes their committees are conducting.