Multiple FBI insiders have claimed the real reason the bureau’s former director James Comey was fired by Donald Trump was because of his refusal to end the investigation into links between Russia and the US leader's presidential campaign team.

The president claimed Mr Comey “wasn’t doing a good job” and documents provided by the White House suggested he was fired for his poor handling of the bureau’s investigation into Hillary Clinton’s use of a private email server.

But speculation has mounted that the director was deposed because of his refusal to end his investigation into allegations that individuals involved with Mr Trump's presidential campaign had undisclosed links to the Kremlin and whether Russian hacking influenced the outcome of last year’s presidential election.

It subsequently emerged that he asked for more resources to fund the investigation, in the days leading up to his firing, although this was denied by the US Department of Justice.

Some commentators and newspapers, including The New York Times, have suggested the President disposed of Mr Comey in a frantic bid to prevent his own impeachment.

Former US attorney for the southern district of New York, Preet Bharara took to Twitter to make a similar claim.

“As a matter of math, infinitely more evidence Comey fired for Russian investigation (some) than there is evidence Trump was wiretapped (zero)”, he wrote.

Mr Bharara, who was removed from his high profile position by Mr Trump in March, added: “Everyone who cares about independence and the rule of law in America should be ‘troubled by the timing and reasoning’ of Comey firing. Period.”

The investigation into the alleged Russian links had been stepped up in recent weeks, with Mr Comey receiving daily instead of weekly updates on the investigation, according to bureau insiders speaking anonymously to the US media.

The Senate’s Intelligence Committee issued a subpoena this week for the President’s former national security adviser Mike Flynn to produce documents relevant to the investigation, which is looking into the nature of his contacts with the Russians before he joined the White House team.

The retired General was forced to resign in February for failing to disclose the content of his talks with Sergei Kislyak, Russia's ambassador to the United States, and then misleading Vice President Mike Pence about the conversations.

Mr Trump insisted the Russia probe wasn’t a factor in his decision to sack Mr Comey.

White House press secretary Sean Spicer also insisted that the President acted “on the clear recommendations” of Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein.

Mr Comey, a New York born lawyer, reportedly refused to give Mr Trump’s aides a preview of his testimony on the investigation into Hillary Clinton’s use of a private email server before a Senate panel, which the President is said to have considered “an act of insubordination”.

Mr Trump, the Attorney General and his deputy had reportedly expected to be given a heads-up about what Mr Comey would say at the hearing which took place on 3 May.

The President said he had had concerns over Mr Comey for some time but it was the FBI’s “clarification” that its former chief had mishandled the investigation into Ms Clinton that prompted the dismissal.

But Democrats and Republicans alike have said the dismissal is concerning and calls into question the independence of the FBI, with several congressman calling for the immediate appointment of an independent counsel to take over the Russian investigation.

Mr Trump has expressed repeated frustration into the enquiry into the alleged Russian ties, insisting it is based on “fake news”. Moscow has denied any official meddling in the US election and the Trump administration denied all allegations of collusion with Russia.

A former Trump adviser said that Mr Comey’s testimony into the Clinton emails had reinforced in the President’s mind that the FBI director was “against him”.

Deputy White House Press Secretary Sarah Huckerbee Sanders told reporters that Mr Trump had been considering firing Mr Comey “since the day he was elected”.

Mr Trump took to Twitter in his customary style to accuse the Democrats of hypocrisy.

“(They) have said some of the worst things about James Comey, including the fact that he should be fired, but now they play so sad.”

He added: “Comey lost the confidence of everyone in Washington, Republican and Democrats alike. When things calm down they will be thanking me!”

Mr Comey has not given any interviews since his dismissal, but said in a farewell letter to his colleagues at the FBI: “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 any time on the decision or the way it was executed. I hope you won’t either. It is done.”

The President has the authority to fire the FBI director but it has only happened once before when Bill Clinton dismissed William sessions after an internal watchdog uncovered financial irregularities.

Amid the gathering storm of protest over the decision, Mr Trump yesterday welcomed Sergey Lavrov, the Russian foreign minister, to the White House.

Mr Lavrov later shrugged off questions about the alleged links between the Trump team and his own government’s officials, comparing the President favourably to his predecessor Barack Obama.

“We discussed our bilateral relations which are not very encouraging. The reason is well known. Unfortunately the previous administration bent over backwards to undermine the solid foundation of those relations.