European Council President Donald Tusk warned Britain on Thursday that its plan to leave the EU’s customs union and single market on Brexit could mean a return to a “hard border” on the island of Ireland. He said an EU proposal on Wednesday to incorporate Northern Ireland within a “common regulatory area” with the EU was the best option to avoid border friction, Reuters said. Tusk made the statement addressing a business conference in Brussels before leaving for lunch in London with British Prime Minister Theresa May. “Until now, no one has come up with anything wiser than that,” Tusk told the Business Europe event. “I will be asking London whether the UK government has another idea that will be as effective in preventing a hard border between Ireland and Northern Ireland.”