President Trump plans to withdraw from a 144-year-old postal treaty that has allowed Chinese companies to ship small packages to the United States at a steeply discounted rate, undercutting American competitors and flooding the market with cheap consumer goods.

The withdrawal, announced by the White House on Wednesday, is part of a concerted push by Mr. Trump to counter China’s dominance and punish it for what the administration says is a pattern of unfair trade practices. The White House, in a statement, said “sufficient progress has not been made on reforming terms” of the postal treaty and that it would begin the withdrawal process while seeking to “negotiate bilateral and multilateral agreements that resolve the problems.”