Allegations Donald Trump revealed highly classified information to Russian officials are "the most serious charge ever made against a sitting president", former Harvard law professor Alan Dershowitz has said.

The legal scholar said people should not minimise the gravity of claims the President revealed the top-secret information about Isis to the Russian foreign minister and ambassador at a White House meeting.

The information is said to have been provided to Mr Trump by an American ally through a sensitive intelligence sharing arrangement, reported The Washington Post.

"This is the most serious charge ever made against a sitting president. Let’s not minimize it. Comey affair now in the wastebasket of history," wrote the professor on Twitter.

Professor Dershowitz, 78, spent more than 50 years at Harvard, becoming the college's youngest ever professor of law in 1967 at the age of 28.

According to the Post, the source of the information had not given permission for the information to be shared with Russia.

After the meeting, White House officials took steps to contain the damage, placing calls to the CIA and National Security Agency. It has not been reported which third country was involved.

The British Embassy in Washington declined to comment when contacted by The Independent.

The White House has denied that the President leaked the classified information to Sergei LAvrov and Ambassador Sergey Kislyak.

In a brief appearance outside the White House, HR McMaster, the president's national security advisor, said: “The story that came out tonight, as reported, is false."

“At no time were intelligence sources or methods discussed,” Mr McMaster said. “And the president did not disclose any military operations that were not already publicly known.

"Two other senior officials who were present, including the Secretary of State, remembered the meeting the same way and said so. The on-the-record accounts should outweigh those of anonymous sources.”

He said he had been present at the meeting.

Donald Trump's first 100 days: in cartoons Show all 33 1 /33 Donald Trump's first 100 days: in cartoons Donald Trump's first 100 days: in cartoons Donald Trump's first 100 days in office were marred by a string of scandals, many of which caught the eye of the Independent's cartoonists Donald Trump's first 100 days: in cartoons Trump's first 100 days have seen him aggressively ramp up tensions with his nuclear rivals in North Korea Donald Trump's first 100 days: in cartoons Mr Trump has warned of a "major, major conflict" with the pariah nation lead by Kim Jong Un Donald Trump's first 100 days: in cartoons Mr Trump dropped the "mother of all bombs" on alleged ISIS-linked militants in Afghanistan, amid an escalation of US military intervention around the globe Donald Trump's first 100 days: in cartoons Mr Trump has been accused of falling short of the standards set by his predecessors in the Oval Office, including Franklin D Roosevelt Donald Trump's first 100 days: in cartoons The tycoon's ascension to the White House came at a time when the balance of power is shifting away from Western nations like those in the G7 group Donald Trump's first 100 days: in cartoons Western politicians, including the British Conservative party, have been accused of falling in line behind Mr Trump's proposals Donald Trump's first 100 days: in cartoons Brexit is seen to have weakened Britain, reducing still further any political will to resist American leadership Donald Trump's first 100 days: in cartoons Mr Trump's leadership has been marked by sudden and unexpected shifts in global policy Donald Trump's first 100 days: in cartoons Trump's controversial missile strike on Syria, which killed several citizens, was seen by some analysts as an attempt to distract from his policy elsewhere Donald Trump's first 100 days: in cartoons The President has also spent a large majority of his weekends golfing, rather than attending to matters of state Donald Trump's first 100 days: in cartoons 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The development was the latest incident to shake Washington, still stunned by the firing of FBI Director James Comey. The White House has been scrambling to project a sense of order, after Mr Trump himself created mayhem by contradicting his own officials explanation for the firing.

The report on Monday evening said one the most concerning aspects of the slip, was that Mr Trump revealed the city in Isis territory where the US intelligence partner detected the threat.