The U.S. has been importing about three million parcels a day with little customs inspection, raising concerns about the ease of sending fentanyl.

The U.S. Postal Service’s decision may block or delay parcels from retailers including Shein and Temu from entering the United States.

However, on Tuesday new rules came into force which closed a loophole that allowed small packages worth $800 (£641) or less to be sent to the US without paying tax or fees. It was one of the measures announced by US President Donald Trump who imposed an additional 10% tariff on all goods imported to the US from China.