(CNN) British Prime Minister Theresa May has told the European Union that the UK will not accept its proposals to deal with one of the thorniest issues in Brexit talks -- the issue of the Irish border.

In a speech in Belfast Friday, May criticized the EU's "backstop" plan, whereby Northern Ireland would remain closely tied to European regulations after Britain leaves the bloc, in the event of the two sides failing to reach an agreement on a future relationship.

The proposal is intended to avoid a "hard" border between Northern Ireland, which will leave the EU as part of the UK, and the Republic of Ireland, which remains in the bloc. Border infrastructure such as customs and security posts were removed as part of the Good Friday Agreement that brought peace to Northern Ireland in the late 1990s.

But, if implemented, it would effectively create a border down the Irish Sea between the island of Ireland and Great Britain. May said that would be unacceptable to any British prime minister. May has proposed instead that any backstop should apply to the whole of the UK.

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"The economic and constitutional dislocation of a formal 'third country' customs border within our own country is something I will never accept and I believe no British prime minister could ever accept," May said in her speech at Belfast's Waterfront Hall.

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