President Trump is expected to announce on Thursday that the United States will strike a trade agreement with Britain, according to three people familiar with the plans.

Mr. Trump teased a new trade agreement in a social media post on Wednesday night, though he did not specify which nation was part of the deal.

“Big News Conference tomorrow morning at 10:00 A.M., The Oval Office, concerning a MAJOR TRADE DEAL WITH REPRESENTATIVES OF A BIG, AND HIGHLY RESPECTED, COUNTRY. THE FIRST OF MANY!!!” he wrote.

A spokesman for the White House declined to comment beyond Mr. Trump’s post. A spokesman for the British Embassy in Washington did not respond to a request for comment.

The agreement would be the first deal announced since Mr. Trump imposed stiff tariffs on dozens of America’s trading partners. He later paused those temporarily in order to allow other nations to reach agreements with the United States.

A deal between the United States and Britain could be a significant win for both countries, which have long sought closer economic cooperation.