The United States will delay its deal to remove tariffs on UK steel and aluminum because of concerns about post-Brexit trade rules affecting Northern Ireland, the Financial Times reported on Wednesday.

The administration of former U.S. President Donald Trump imposed 25% and 10% tariffs on steel and aluminum imports on the European Union in 2018. The tariffs were withdrawn in October of this year, but they remain in place for Britain due to its exit from the EU.

In a communication seen by the newspaper, a U.S. Commerce Department official was quoted as saying that talks with the UK on easing metals tariffs could not move ahead.

The official cited U.S. concerns about British threats to trigger emergency clause Article 16, the report said , especially from the U.S. Congress.