Trade between the United Kingdom’s four nations could be disrupted if Scotland wins a different divorce deal with the European Union, a letter from Brexit minister David Davis published by the Scottish government said on Friday.

Scotland last December proposed a separate trade carve-out for itself in London’s upcoming divorce talks with the EU.

The core of Scotland’s proposal is for it to remain fully within the EU’s single market for goods and services, whereas British Prime Minister Theresa May wants to take the U.K. out of it to restore control over immigration.