Donald Trump's lawyers understand that obstruction of justice charges against the President are “real and serious", a former US Attorney has said.

In addition to four congressional investigations, the US President and his 2016 campaign are under investigation by special counsel and former FBI director Robert Mueller, over claims they colluded with Russia.

Mr Trump’s lawyers have met several times with Mr Mueller over recent months and have submitted detailed memos rebutting the allegations.

And Former US Attorney Preet Bharara said the fact the President's legal representatives had given written responses to the claims showed they recognised their significance.

"There may never be any charges, but defense lawyers don't usu waste time on preemptive memos re: frivolous theories of criminal guilt," he wrote on Twitter.

"The possibility that POTUS may have obstructed justice is real and serious. Apparently, even Trump's defense lawyers get that."

Former South Alabama State Attorney Joyce Alene agreed with Mr Bharara's view, saying "most defence lawyers wouldn't dignify a prosecutor's allegation of a specific crime w/a written response unless charges were close."

Mr Bharara was US Attorney for the Southern District of New York from 2009 to 2017, until he was sacked by the Trump administration in March after refusing to be forced to resign along with 46 other US Attorneys – despite having been asked to stay on as recently as November last year.

US Attorneys represent the federal government as prosecutors, a senior legal position, and Mr Bharara was known for his aggressive pursuit of both financial crime and political corruption in New York.

Shortly after his removal, it emerged Mr Bharara had been investigating healthcare stock trades made by Mr Trump’s health and human services secretary, Tom Price, as he was voting on laws that would effect the industry. He was also the prosecutor in the trials of several of disgraced financier Bernie Madoff's senior employees.

Since being fired, Mr Bharara has been one of several legal figures offering running commentary on social media of the President's legal woes.

The news of legal arguments and meetings, as reported by the Wall Street Journal, offer the first detailed information on the interactions between Trump's team and their investigators.

One memo from Mr Trump's legal team, written in June, made the case that the President has the authority to appoint and dismiss staff as he sees fit, and that therefore there was no constitutional basis for charges of obstruction of justice in his firing of former FBI director James Comey.

Another memo submitted the same month attempted to suggest Mr Comey would make an unsuitable witness in any inquiry, calling him prone to exaggeration, unreliable in congressional testimony and the source of leaks to the media.

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Mr Comey was initially heading up the investigation, but after his sacking in May, Mr Mueller was appointed by the Justice Department with a wide brief to investigate not just possible coordination but “any matters” that arose from the investigation.

That includes whether Mr Trump obstructed justice by attempting to alter the course of the investigation.