The FBI secretly launched an investigation into whether Donald Trump was working on behalf of Russia while in office, according to an explosive report by The New York Times.

The decision was said to have been taken after Mr Trump fired James Comey, the FBI director leading the Russia election meddling investigation at the time, in May 2017.

FBI agents had been weighing up the move for months but decided to act when Mr Trump linked the firing to the relief it would bring from the Russia probe, according to the paper.

The move would have put the FBI in an extraordinary position - having to consider whether the US president himself posed a national security risk to the country he was leading.

The investigation reportedly was only a few days old when Robert Mueller was appointed special counsel to take over the Russia investigation. It is unknown whether he is still pursuing that line of inquiry.

Mr Trump on Saturday morning sent six tweets hitting out at the Russia probe. Referencing the New York Times article, the US president said FBI leaders had launched the investigation “for no reason and with no proof”.