Donald Trump leak allegations the 'most serious charge ever made against a sitting president', says former Harvard law professor

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.

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What did Trump say? Everything we know about the President's leak

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.

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.

There were claims that Russia could use the information to undermine the US, or its ally.