WASHINGTON (Reuters) - President Donald Trump has the right to instruct advisers not to testify before congressional oversight probes related to the Russia investigation, the White House said in a letter that blasts Special Counsel Robert Mueller’s report as deeply flawed.

FILE PHOTO: Robert Mueller, as FBI director, testifies before a Senate Intelligence Committee hearing on Capitol Hill in Washington March 12, 2013. REUTERS/Kevin Lamarque/File Photo

The April 19 letter from White House legal counsel Emmet Flood to Attorney General William Barr, obtained by Reuters on Thursday, was in line with Trump’s confrontational approach to dealing with a Democratic effort to use the Mueller report as a springboard into more investigations.

Flood said Trump’s decision to let advisers cooperate with the Mueller probe does not extend to congressional oversight investigations. Democrats have argued Trump waived the right to assert executive privilege by allowing advisers to cooperate extensively with Mueller.

The White House conclusion was an indication that Trump would, if necessary, declare executive privilege to prevent former lawyer Don McGahn and other advisers from testifying to Congress, which would likely trigger a court battle.

“It is one thing for a president to encourage complete cooperation and transparency in a criminal investigation conducted largely within the Executive Branch. It is something else entirely to allow his advisers to appear before Congress...” the letter said.

Tensions have flared between the White House and congressional Democrats since the Mueller report was released. Jerrold Nadler, chairman of the House of Representatives Judiciary Committee, threatened to hold Barr in contempt for resisting demands for Mueller’s full, unredacted report, and accused Trump of “a growing attack” on democracy.

House Speaker Nancy Pelosi, the top Democrat in Congress, accused Barr of committing a crime, saying he lied to lawmakers about his interactions with Mueller.

The White House letter was transmitted to Barr a day after the release of the Mueller report, in which the special counsel determined that Trump did not collude with Russia during the 2016 presidential campaign but pointedly did not say whether the president committed obstruction of justice.

Flood’s letter said the Mueller report itself suffers from “an extraordinary legal defect.” According to relevant law, the letter said, Mueller should have rendered a judgment on whether to prosecute or not to prosecute.

Instead, Mueller produced “a prosecutorial curiosity - part ‘truth commission’ report and part law school exam paper,” the letter said.

“What prosecutors are supposed to do is complete an investigation and then either ask the grand jury to return an indictment or decline to charge the case,” Flood wrote in his the five-page letter to Barr.

“The Special Counsel and his staff failed in their duty to act as prosecutors and only as prosecutors,” he said.

Flood’s letter addressed whether Mueller had given Congress a “roadmap” for its own inquiry with his report.

If that is the case, the letter said, “it too serves as additional evidence of the SCO’s (Special Counsel’s Office) refusal to follow applicable law.”

The letter also provided a potential justification for a probe that Trump wants into those who began investigating his possible ties to Russia in 2016, when he was still a candidate.

Of concern in particular were leaks about conversations that former Trump foreign policy adviser Michael Flynn had with Sergey Kislyak, the U.S. ambassador to Russia, in 2016.

“Government officials, with access to classified information derived from a counterintelligence investigation and from classified intelligence intercepts, engaged in a campaign of illegal leaks against the president,” the letter said.