Donald Trump would most likely fight any subpoena to testify before the Russia probe all the way to the Supreme Court, his personal lawyer has said.

Special counsel Robert Mueller has been in negotiations with the president’s legal team about questioning about his campaign’s alleged collusion with Russia during the 2016 election. The president has said – at least in public – he wants to sit down and talk with Mr Mueller and his lawyer said the ultimate decision would be Mr Trump’s.

“If Mueller were to try to compel the president’s testimony, there would be a legal fight over the constitutionality of requiring such an act of a sitting president, Mr Trump’s lawyer Jay Sekulow, told ABC News.

“A subpoena for live testimony has never been tested in court as to the president of the United States.”

In January, Mr Trump said he was prepared to testify under oath, claiming he believed any such session would clear him of any suspicions he had been involved in colluding with Russia.

“I’m looking forward to it actually,” Trump told reporters in the West Wing. “There has been no collusion whatsoever, there has been no obstruction whatsoever.”

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Yet, his various lawyers have always been opposed to the idea, fearful the president may not retain the sort of discipline they would require of him if he were to sit down with Mr Mueller.

Rather, they have playing for time and instead exploring the possibly of responding to written questions. Last week, it was reported by the New York Times Mr Mueller had told the president’s lawyers he would agree to written responses, if in-person follow questions were permitted.

Mr Trump’s lawyers reportedly disliked this idea, but the president told his team to keep negotiating, convinced that he could show Mr Mueller he did nothing wrong.

If Mr Trump refused to agree to questioning, Mr Mueller could try and obtain a subpoena, forcing him to appear. But Mr Sekulow claimed the president had the authority under Article II of the US Constitution to stop any investigation conducted by the Department of Justice. The article enumerates the powers of the executive branch of the federal government, Reuters said.

One part of Mr Mueller’s inquiry and congressional investigations has focused on a meeting months before the November election in Trump Tower in New York between Russian officials, Donald Trump Jr, the president’s eldest son, and other campaign aides.

Trump in Helsinki: Putin meeting and protests Show all 16 1 /16 Trump in Helsinki: Putin meeting and protests Trump in Helsinki: Putin meeting and protests President Trump meets with Russian President Vladimir Putin in Helsinki Reuters Trump in Helsinki: Putin meeting and protests An advert from Finnish newspaper Helsingin Sanomat is on display in Helsinki Reuters Trump in Helsinki: Putin meeting and protests President Trump in a meeting with Finnish President Sauli Niinisto AFP/Getty Trump in Helsinki: Putin meeting and protests President Trump meets with Finnish President Sauli Niinisto in Helsinki EPA Trump in Helsinki: Putin meeting and protests President Trump talks with Finnish President Sauli Niinisto in Mantyniemi, the official residence of the Finnish President EPA Trump in Helsinki: Putin meeting and protests US, Finnish and Russian flags fly in front of the Presidential Palace in Helsinki EPA Trump in Helsinki: Putin meeting and protests A protester wears a mask featuring a portrait of Russian President Vladimir Putin during the so-called "Helsinki against Trump and Putin" demonstration on July 16, 2018, in the Finnish capital Helsinki. The US and Russian leaders opened an historic summit in Helsinki on Monday, with Donald Trump promising an "extraordinary relationship" and Vladimir Putin saying it was high time to thrash out disputes around the world. / AFP PHOTO / Jonathan NACKSTRANDJONATHAN NACKSTRAND/AFP/Getty Images JONATHAN NACKSTRAND AFP/Getty Trump in Helsinki: Putin meeting and protests Vladimir Putin gifts a football to President Trump at the press conference that followed their meeting AP Trump in Helsinki: Putin meeting and protests Trump supporters hold banners during a demonstration in Helsinki AFP/Getty Trump in Helsinki: Putin meeting and protests People protest for women's reproductive rights in Helsinki's Senate Square Reuters Trump in Helsinki: Putin meeting and protests A crowd watches the motorcade transporting President Trump through Helsinki AFP/Getty Trump in Helsinki: Putin meeting and protests Russian President Putin drives through Helsinki on his way to meet with President Trump EPA Trump in Helsinki: Putin meeting and protests A man in the audience of the joint press conference holds up a sign sign that reads "NUCLEAR WEAPON BAN TREATY". REUTERS Trump in Helsinki: Putin meeting and protests A man is removed from the joint press conference in Helsinki. Security removed the man after he pulled out a sign that read "NUCLEAR WEAPON BAN TREATY". REUTERS Trump in Helsinki: Putin meeting and protests President Trump and Russia's President Putin sit for a working lunch in Finland's Presidential Palace AFP/Getty Trump in Helsinki: Putin meeting and protests President Trump meets with Russian President Vladimir Putin in Helsinki Reuters

While originally Mr Trump said the meeting was about adoptions, on Sunday he said on Twitter that it was about getting information on his election opponent, Democrat Hillary Clinton.

Mr Trump and his son have both said then-candidate Trump had no advance knowledge of the meeting and that the meeting itself was not useful to the campaign. CNN reported last month that Michael Cohen, the president’s longtime personal lawyer and self-described “fixer“ was willing to tell Mr Mueller that Mr Trump did know about the meeting in advance.

On Sunday morning, Trump responded to reports in the Washington Post and on CNN that he was concerned Donald Jr could be in legal trouble.

He wrote: “Fake News reporting, a complete fabrication, that I am concerned about the meeting my wonderful son, Donald, had in Trump Tower. This was a meeting to get information on an opponent, totally legal and done all the time in politics – and it went nowhere. I did not know about it!”

Mr Trump has always denied colluding with Russia during the 2016 election. Russian president Vladimir Putin has likewise denied his nation interfered in the process.