European Council leader Donald Tusk said there is a "real" danger of the European Union breaking up over a deal to keep Britain in the union, as well as the strain of mass migration.
"This is a critical moment," Tusk said in Bucharest, Romania, just days ahead of a key EU summit.
"It is high time we started listening to each other's arguments more than our own. The risk of a break-up is real because this process is indeed very fragile and must be handled with care," Tusk said.
"What is broken cannot be mended," he added.
Tusk was in Bucharest as part of a tour of key EU capitals in an effort to bring about consensus at a crucial EU summit on Thursday and Friday.
The summit is expected to focus on Britain's demands to renegotiate its terms of membership with the European Union.
Also on the agenda is the wave of migrants that has arrived on the European continent in the past year. Tusk said the mass influx is "stretching our union to its limits."
Some information from Reuters, AP and AFP.