Outgoing FBI Director James Comey has used his resignation letter to urge former colleagues at the bureau to continue “upholding the constitution”.

In the letter, Mr Comey also said he would not dwell on the decision and hoped others would not either, despite the brutal method by which he was dispatched.

He wrote: “I have long believed that a President can fire an FBI Director for any reason, or for no reason at all.

“I’m not going to spend time on the decision or the way it was executed. I hope you won’t either.”

The short note released on Wednesday evening continued: "My hope is that you will continue to live our values and the mission of protecting the American people and upholding the constitution."

Mr Comey's surprise dismissal on Tuesday stunned Washington and plunged Donald Trump deeper into a controversy over his campaign's alleged ties with Russia.

The President claimed the sacking was the result of poor decisions made by Mr Comey while he was at the helm of the FBI.

But critics were quick to question why Mr Trump had waited until now – more than 100 days into the presidency – to sever the erstwhile director’s contract.

The measured tone of Mr Comey’s remarks contrasted with claims from the White House of “atrocities” carried out by the FBI chief while he was in charge.

White House spokeswoman Sarah Huckabee Sanders referred to what she called "atrocities in circumventing the chain of command" at the Justice Department in attempting to explain his firing.