BRUSSELS (AP) — European Union Council President Donald Tusk said Saturday that Britain will face a united bloc of 27 EU nations in its divorce negotiations and warned that any demands from Prime Minister Theresa May will be dealt with "firmly."

Tusk's remarks came before he chairs a summit of 27 EU leaders in Brussels to finalize the cornerstones of their negotiating stance after May triggered two years of exit talks on March 29. The negotiations themselves are to start shortly after snap elections in Britain on June 8.

Tusk insisted that there can be no discussions on a future relationship before some key issues like how much Britain owes the bloc are sufficiently agreed, and he said that the welfare of citizens and families living in each other's nations will be the priority once the talks start.

All sides "need solid guarantees for all citizens and their families who will be affected by Brexit on both sides. This must be the No. 1 priority," Tusk said Some 3 million citizens from the 27 nations live in Britain while up to 2 million Britons live on the continent, all facing uncertainly on such issues as welfare and employment.

Tusk said that sustained unity of the 27 will help May since she will have political certainty throughout the talks.

"Our unity is also in the U.K.'s interest," he said. "I feel strong support from all the EU institutions, including the European Parliament, as well as all the 27 member states. I know this is something unique and I am confident it will not change."

Over the past years, the bloc has often been bitterly divided over issues like the financial crisis and how to deal with hundreds of thousands of migrants entering the bloc.

At their summit, the leaders are set to acknowledge that Northern Ireland could join the bloc in the future if its people vote to unite with EU member state Ireland.

Two European officials said a statement on the issue is likely to be added to the minutes of the summit, to be held without May. The officials asked not to be identified because the summit of 27 leaders still was being prepared.

Future relations between Ireland and Britain, including how the border between Ireland and Northern Ireland would work with the U.K. outside the bloc, have emerged as a key problem to be addressed during the Brexit talks.