A Justice Department official on Monday wrote a letter to former Special Counsel Robert Mueller informing him that he needs to stick to the public findings in his report.
Mueller’s testimony, the letter stated, "must remain within the boundaries of your public report because matters within the scope of your investigation were covered by executive privilege, including information protected by law enforcement, deliberative process, attorney work product, and presidential communications privileges."
The document, signed by Associate Deputy Attorney General Bradley Weinsheimer, also said the privileges “would include discussion about the investigative steps or decisions, made during your investigation not otherwise described in the public version of your report. Consistent with standard practice, Department witnesses should decline to address potentially privileged matters, thus affording the Department the full opportunity at a later date to consider particular questions and possible accommodations that may fulfill the committees' legitimate need for information while protecting Executive Branch confidentiality interests."
Mueller, who is scheduled to testify before Congress about the Russia probe in just a few days, wrote to the DOJ requesting guidance about his testimony.
Mueller is appearing before the committee by subpoena.
Mueller completed in March his nearly two-year-long probe into Russian meddling in the 2016 election and possible obstruction of justice by President Donald Trump.
The Justice Department released a redacted copy of his 448-page report in April.
A spokesman for Mueller, Jim Popkin, said no one at the Justice Department, Congress or the White House would review Mueller's statement before he delivers it on Wednesday.
In back-to-back hearings before the House of Representatives Judiciary and Intelligence committees, Democrats are expected to try to get Mueller to focus his testimony on specific examples of Trump's misconduct.
The chairman of the Judiciary Committee, Democratic Representative Jerrold Nadler, said on Sunday he believed there was "substantial evidence" that Trump committed high crimes and misdemeanors.
Nadler's comments are significant because evidence of such crimes would be required if Democrats pursue impeachment proceedings against the president.
Trump has denied any wrongdoing and repeatedly slammed the Mueller investigation as a "witch hunt."
Mueller said in a statement on May 29 he would not go beyond what was in the report in any testimony to Congress.
The Mueller report laid out numerous contacts between Russian officials and Trump's campaign, but found no evidence of a criminal conspiracy.
It also gave examples of 10 incidents in which Trump sought to hinder the investigation, but it did not draw any conclusions on whether Trump obstructed justice. Attorney General William Barr later concluded he did not see enough evidence to bring obstruction charges.
Mueller has been using offices at his former law firm WilmerHale and working with a small team from the special counsel's office to prepare for Wednesday's hearings, Popkin said.
"He will come well prepared," Popkin said. "His team has been working on this for a while and they will be ready for whatever comes their way."
Reuters contributed to this report.