Ruth Sherlock, our US editor, writes:

Donald Trump's decision to fire FBI director James Comey is simply extraordinary.

It comes as Mr Comey is leading an investigation into whether members of President Trump’s campaign colluded with Russia to influence the 2016 election.

In so doing Mr Comey brought a criminal investigation to the doorstep of the White House, and vowed to pursue it “no matter how long that takes.”

Parallels are already being drawn to the "Saturday Night Massacre" of 1973, where, in the midst of the Watergate scandal, then President Richard Nixon, dismissed Archibald Cox, the independent special prosecutor.

And he accepted the resignations of Elliot L. Richardson, the attorney general, and William Ruckelshaus, the Deputy Attorney General, after both men refused to accept a deal, related to Watergate, that Mr Nixon was seeking to strike.

Whilst Mr Trump has the constitutional power to fire Mr Comey, there is no escaping the fact that with this decision he has removed the very person investigating him and his allies.

Mr Comey's most controversial actions - his decision to announce a renewed investigation into Hillary Clinton's emails only days before the elections, and mistakes in his testimony to Congress - have all been to the detriment of the former Democratic presidential nominee.

Why then Mr Trump decided to take this sweeping action is a key question that his administration will need to answer.