The DUP have described the EU draft deal to protect the open border in Ireland by keeping the north in the customs union as "unacceptable".
The 120-page document states, "the territory of Northern Ireland, excluding the territorial waters of the United Kingdom ... shall be considered to be part of the customs territory of the Union".
Customs controls would be "carried out jointly by the Union and the United Kingdom customs authorities competent for the territory of Northern Ireland".
Northern Ireland would remain subject to EU restrictions restricting state aid for companies, in respect of measures affecting trade between the territory and the EU.
The proposals sparked anger from the DUP, with leader Arlene Foster calling it "constitutionally unacceptable."
EU draft text is constitutionally unacceptable & would be economically catastrophic for Northern Ireland. I welcome the Prime Minister's commitment that HMG will not allow any new border in the Irish Sea. Northern Ireland must have unfettered access to GB market. AF
— Arlene Foster (@DUPleader) February 28, 2018
Speaking this morning after details of the document were leaked, North Antrim MP Ian Paisley told the NI Affairs Committee: "I'm disgusted by what the EU are saying about my country. I'm appalled."
Mr Paisley said the British government was facing its biggest test of resolve in the Brexit negotiations and described the plan for common regulations between Northern Ireland and the Republic as an attempt to annex part of the UK.
He told Secretary of State Karen Bradley: "I would ask the government to show some teeth to the EU that we will not be rolling over to the demands to annex part of our country."
MEP Diane Dodds called the proposals "economically disasterous" and called for the north to leave the EU "on the same basis as the rest of the UK."
Lets reiterate, yet again, the facts. Northern Ireland's prosperity is more dependent on Britain than on the RoI, rest of the EU and rest of the world combined. Barnier's proposal would be economically disastrous. pic.twitter.com/2QmsDTzW7r
— Diane Dodds MEP (@DianeDoddsMEP) February 28, 2018
Lagan Valley MP Jeffrey Donaldson said if the EU or Dublin government believed the document would be acceptable to the British government "they are deluded."
This fundamentally breaches the understanding reached in December and would undermine the constitutional status of NI in the Belfast Agreement. If the EU or Dublin believes the UK Government will be signing up to a border in the Irish Sea, they are deluded. Taoiseach knows that. https://t.co/77dUekdO0H
— Jeffrey Donaldson MP (@J_Donaldson_MP) February 27, 2018
Speaking during British Prime Minister's Questions, Theresa May said the EU document proposes the creation of a border in the Irish Sea.
"No UK prime minster could ever agree to it," she said.
"The draft legal text the Commission have published would, if implemented, undermine the UK common market and threaten the constitutional integrity of the UK by creating a customs and regulatory border down the Irish Sea, and no UK prime minster could ever agree to it.
"I will be making it crystal clear to President Juncker and others that we will never do so."
The DUP's MPs prop up the Conservative government at Westminster. Theresa May is due to present her vision for Britain's future relationship with the EU in a speech on Friday.