President Trump said on Friday that he would ban CNN, MS NOW and Politico from the White House. Mr. Trump published his abrupt proclamation, which he said was “effective immediately,” in a post on Truth Social, the social media platform he owns. The president wrote that his move was “a result of their constant ‘reporting’ FAKE NEWS!,” repeating one of his most common complaints about independent news outlets.

It was not clear how such a ban might be enforced, or whether it could pass muster in the event of a legal challenge. The White House did not immediately respond to a request for comment about Mr. Trump’s post. Representatives of the three news organizations mentioned by Mr. Trump also did not immediately respond to a request for comment.

Mr. Trump has tried to bar some news organizations from parts of the White House in the past. During his first term, he revoked a White House pass for Jim Acosta, the former CNN journalist. CNN succeeded in getting that ban overturned in court. This is a developing story. Check back for updates.