Donald Trump has marked the one-year anniversary of the special counsel’s Russia investigation with a heated early morning tweetstorm calling it “disgusting” and “illegal”.

In a mocking message on Twitter, the US president wrote: “Congratulations America, we are now into the second year of the greatest Witch Hunt in American History.”

He also repeated claims that a “confidential informant” had “spied” on the Trump campaign before the election for the FBI, declaring: “If so, this is bigger than Watergate!”

The accusation appears to be linked to a suggestion by the man who commissioned the Christopher Steele dossier on Mr Trump’s Russia ties that someone in the campaign became a whistleblower.

A string of new details emerged in the US media on Thursday in pieces marking the one-year mark of Robert Mueller’s appointment as special counsel.

He was tasked with leading the Russian election-meddling investigation after James Comey, the FBI director, was fired by the president.