The Justice Department opened a new front in the legal battle between congressional impeachment investigators and the White House on Tuesday by announcing that Congress must allow government attorneys to accompany executive branch witnesses who testify about President Trump‘s relations with Ukraine.

In a newly released memo, the Justice Department’s Office of Legal Counsel said “the assistance of agency counsel” is needed because testimony has the potential to disclose information “protected by executive privilege.”

The House Intelligence Committee, which has taken the lead on the impeachment inquiry, has heard from a number of witnesses whose personal lawyers have been allowed to attend depositions. But the Justice Department argued in its memo that the exclusion of government lawyers deprives Trump of his constitutional power to screen privileged information from lawmakers.