President Donald Trump confirmed on Friday for the first time that he was under investigation in the expanding inquiry into Russian influence in the election as he apparently let his frustration spill out on to Twitter.

In an early-morning post, he acknowledged he was being investigated for dismissing the FBI director and then turned his fire on his deputy attorney general for leading what he described as a “witch hunt”.

Mr Trump used Twitter to powerful effect during his election campaign, bypassing the established media to reach voters directly. But he now runs the risk of providing evidence to the slew of investigations arrayed against him.