Bloomberg News reporters have been barred from covering events on President Donald Trump’s re-election bid due to alleged “biases”, campaign chiefs said on Monday.

The decision to no longer give credentials to Bloomberg reporters comes a week after the news service’s founder, billionaire Michael Bloomberg, announced he was seeking the Democratic nomination for president.

Bloomberg News, which the former New York City mayor founded in 1990, has also said it will not investigate Mr Bloomberg or his Democratic rivals but would continue to probe the Trump administration, as the sitting government.

Mr Trump’s campaign manager Brad Parscale called it a troubling decision to “formalise preferential reporting policies”, and said Bloomberg reporters would no longer be credentialed to cover campaign events until the policy is rescinded.

“As President Trump’s campaign, we are accustomed to unfair reporting practices, but most news organisations don’t announce their biases so publicly,” Mr Parscale said.

Bloomberg Editor-in-Chief John Micklethwait said the accusation of bias could not be further from the truth.

“We have covered Donald Trump fairly and in an unbiased way since he became a candidate in 2015 and will continue to do so despite the restrictions imposed by the Trump campaign,” he said.

The Trump campaign’s action illustrates the difficult position Bloomberg’s candidacy has imposed on the news organisation.

By saying reporters could not investigate Bloomberg or his Democratic rivals, some critics have said this would prevent the news organisation from conducting in-depth reporting on the campaign. Bloomberg officials say it is a position the news service has navigated before, when Mr Bloomberg was mayor.

“This is my nightmare come true,” said Kathy Kiely, a University of Missouri journalism professor who quit as Bloomberg political director when he was considering a run for the 2016 presidential nomination.

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

Journalists at Bloomberg would have been better served if he had made clear he was stepping away from his company for the campaign, Ms Kiely said, adding Mr Bloomberg — and any candidate for president — was fair game for any kind of stories that Bloomberg News reporters could dig up.

“It’s unfortunate that this is creating a perception that this is how journalism works, that journalists are manipulated by their bosses,” she said.

In a memo sent to staff members following Bloomberg’s announcement, Micklethwait said he would continue the organisation’s policy of not investigating Bloomberg, his family or his foundation, and “will extend the same policy to his rivals in the Democratic primaries”.

If Mr Bloomberg was chosen as the candidate against Mr Trump, the policy would be reevaluated, Micklethwait said at the time.

Dean Baquet, executive editor of The New York Times, also criticised the Trump campaign’s move.

“Bloomberg News is one of the largest and most influential news organisations in the world,” Baquet said. “We condemn any action that keeps quality news media from reporting fairly and accurately on the presidency and the leadership of the country.”