President Trump has instructed his new National Security Adviser to make substantial staffing cuts to the 300-member National Security Council, according to reports — after years of Republican complaints of agency bloat and overreach.

Two administration sources told Bloomberg News the order was timed to the arrival of National Security Adviser Robert O’Brien, who took over Sept. 18 from the ousted John Bolton.

But the move comes as the administration grapples with an impeachment investigation centered on NSC activity — an inquiry touched off by a whistleblower who is rumored to have worked on the NSC’s Ukraine desk during the Obama administration.

On Friday, Trump retweeted a post from investigative reporter Paul Sperry alleging that the whistleblower was involved in Ukraine-based investigations into Trump during the 2016 presidential campaign.

“Where is the LameStream Media on this?” the president asked.

The NSC staff, which numbered around 100 during the Clinton and Bush administrations, swelled to 400 full-time members under President Obama.

In 2016, Republicans in both houses of Congress introduced bills that would have slashed the NSC staff to no more than 150, legislation the Obama administration opposed.

Trump’s order will be carried out mainly by attrition as staffers rotate back to their original positions at the State Department and other federal agencies, and not by layoffs or firings.