WASHINGTON — The Justice Department’s lead antitrust official will not participate in the review of Comcast’s proposed $45 billion takeover of Time Warner Cable, the department said on Thursday.

The official, William J. Baer, assistant attorney general and head of the antitrust division, will not participate in considering the merger because he represented NBCUniversal when it was taken over by Comcast in 2011.

Obama administration guidelines say that during the first two years in office, an appointee is forbidden from taking part in a decision involving a former client. The rules apply to clients represented in the two years before the appointee’s confirmation.

Mr. Baer was confirmed on Dec. 30, 2012. A notice documenting his representation of General Electric and NBC was filed in federal court on Jan. 20, 2011, within two years of his confirmation.