The Democratic National Committee reportedly “did not allow” the Federal Bureau of Investigation to examine its hacked computer servers before the intelligence agency issued a report blaming Russia as the culprit.

“The FBI repeatedly stressed to DNC officials the necessity of obtaining direct access to servers and data, only to be rebuffed until well after the initial compromise had been mitigated,” Buzzfeed reported an FBI statement.

“This left the FBI no choice but to rely upon a third party for information. These actions caused significant delays and inhibited the FBI from addressing the intrusion earlier.”

The third party tech company was CrowdStrike, which said it first discovered the hack in May.

A DNC spokesman previously told the publication that the FBI had not requested access to the hacked servers, and declined to comment after the FBI gave a statement.

In what has become the biggest White House scandal since Watergate, it was discovered that no US government entity had run an independent analysis on the DNC's systems since last summer.

Mr Trump jumped on the news, tweeting further doubt about whether Russia was involved.

Trump's advisor suggests Obama's sanctions against Russia are to 'box in' the incoming President

He is due to receive an in-depth briefing on Russia’s alleged involvement on Friday.

“The Democratic National Committee would not allow the FBI to study or see its computer info after it was supposedly hacked by Russia,” he said.

“So how and why are they so sure about hacking if they never even requested an examination of the computer servers? What is going on?”

In a 13-page report in December, the FBI and the Department of Homeland Security concluded that Russia intended to manipulate the election to boost Mr Trump into the White House, but the report does not specifically mention that they examined DNC computers themselves.

The CIA also concluded that Russia was behind the attack.

Barack Obama: US will act on election hacking by Russia

Less than 24 hours before Mr Trump was due to announce the results of his briefing on Russia and share his own views, his top adviser and the former CIA director James Woolsey quit his transition team.

President-elect Trump also questioned why NBC gained access to information in a classified report on Russia that was presented to president Barack Obama on Thursday.

“Who gave them this report and why? Politics!” he tweeted.

Two intelligence officials told NBC that the report given to Mr Obama, which has not been made public, detailed how Russian operatives had not only infiltrated the DNC systems but also those of the White House.

Hacking was also evident, it found, in 2008 and 2012. The most recent hacking was “payback” for Mr Obama’s questioning of Vladimir Putin’s legitimacy as Russian president.

Mr Obama expelled dozens of Russian diplomats and imposed sanctions on the country following the report from intelligence agencies.