President Trump signed a memorandum on Tuesday directing his administration to revive a decades-old policy that is likely to sharply restrict advertising of prescription drugs on television.

The move reflects one of the top priorities of the health secretary, Robert F. Kennedy Jr., who has repeatedly called for a ban on drug advertising on television. The policy change threatens to dent the revenues of pharmaceutical companies.

The memorandum also stands to hit major television networks, which earn substantial revenue from pharmaceutical advertisers trying to reach older viewers.

The proposal, which would effectively reverse a 1997 policy change that opened the floodgates to a deluge of TV drug advertising, is likely to be aggressively opposed by the drug industry, which has long had the courts on its side on this issue.

Past efforts to even modestly restrict drug advertising have been blocked by the courts on First Amendment grounds. The White House said it planned to change the policy via a rule-making process.

Dr. David Kessler, who as commissioner of the Food and Drug Administration in the 1990s opposed loosening restrictions on drug advertising, said, “The approach that they are proposing to follow would in essence remove direct-to-consumer advertising from television.”