Bloomberg News reporters were denied on Monday news coverage of President Donald Trump's alleged "prejudice".

One week after billionaire Michael Bloomberg, the founder of the intelligence service, announced his intention to nominate the Democrats as president, it is decided not to provide credentials to Bloomberg reporters.

Bloomberg News, founded in 1990 by the former mayor of New York, has also announced that he will not launch an investigation against Mr Bloomberg or his Democratic rivals, but will continue to investigate the Trump administration as a session government.

Brad Parscale, Trump's campaign manager, said it was a disturbing decision to "formalize preferred reporting policies" and said Bloomberg reporters would no longer be eligible to report on campaign events until the policy was repealed.

"As President Trump's campaign, we are used to unfair reporting practices, but most news organizations do not share their prejudices so publicly," Parscale said.

John Micklethwait, editor-in-chief of Bloomberg, said the accusation of bias could not be further from the truth.

"We have treated Donald Trump fairly and impartially since becoming a candidate in 2015 and will continue to do so despite the restrictions imposed by the Trump campaign," he said.

The action of the Trump campaign illustrates the difficult position that Bloomberg's candidacy has imposed on the news organization.

Some critics said that reporters could not investigate Bloomberg or his Democratic rivals, which would prevent the news organization from reporting extensively on the campaign. Bloomberg officials say it is a position that the intelligence service had previously taken as mayor of Bloomberg.

"This is my nightmare," said Kathy Kiely, a journalism professor from the University of Missouri, who announced as Bloomberg's political director, thinking about running for the 2016 presidential election.

Democratic presidential candidate Michael Bloomberg talking to reporters recently in Phoenix (Rick Scuteri / AP)

The Bloomberg journalists would have been better off if he made it clear that he was withdrawing from his company for the campaign, said Kiely, adding that Mr Bloomberg – and every presidential candidate – for any kind of story, the Bloomberg news reporter write that a fair game could be digging.

"It is unfortunate that it creates the perception that journalism works to manipulate journalists by their bosses," she said.

In a memo to employees following Bloomberg's announcement, Micklethwait said he would continue the organization's policy of not investigating Bloomberg, his family, or his foundation and "extending the same policy to his rivals in the Democratic primary."

If Mr. Bloomberg were elected as a candidate against Mr. Trump, politics would be re-evaluated, Micklethwait said at the time.

Dean Baquet, editor-in-chief of the New York Times, also criticized the move of the Trump campaign.

"Bloomberg News is one of the largest and most influential news organizations in the world," said Baquet. "We condemn all measures that prevent high-quality news media from reporting fairly and accurately on the country's presidency and leadership."