European Union leaders at a Brexit summit will give a formal undertaking to embrace the British province of Northern Ireland in the EU if a referendum unites the island, diplomats said.

Irish Prime Minister Enda Kenny has asked fellow members of the bloc to acknowledge that Northern Ireland would, like East Germany in 1990, automatically enter the EU in the event of unification with existing member state, the Irish Republic.

The 1998 Good Friday Agreement to end violence in the north foresees the holding of referendums on both sides of the Irish border on uniting the island if London and Dublin see public support for such a change.

EU leaders, who will be meeting in Brussels on Sunday to endorse a negotiating plan for Britain's withdrawal, will give a political endorsement to what Irish and EU legal experts say is the position in international law of such territorial changes.

"The European Council acknowledges that the Good Friday Agreement expressly provides for an agreed mechanism whereby a united Ireland may be brought about through peaceful and democratic means; and, in this regard, the European Council acknowledges that, in accordance with international law, the entire territory of such a united Ireland would thus be part of the European Union," a draft text of the declaration reads.

One EU source said the text, to be entered into the formal minutes of the meeting, was a statement of "the obvious" and, along with Irish officials, he stressed the summit was not taking a view on unification or launching any talks on unity.

"Irish unity is not part of the Brexit negotiations but given the importance of the Good Friday Agreement it will be suitable for that to be acknowledged by the European Council," a senior Irish official in Brussels said.

"This is not about starting a process but it is important that there be clear acknowledgement that this is the case."