The president of the European Council says talks on reforms to keep Britain in the European Union are difficult but that he hopes an offer can be made to the country in February.

Donald Tusk said in a letter to EU leaders on Monday that "we should be able to prepare a concrete proposal to be finally adopted in February."

Britain will hold a referendum by the end of 2017 to decide whether to leave the 28-nation bloc. Cameron wants to stay in if he can secure certain reforms.

They include giving member states more autonomy and allowing Britain to restrict benefits for EU immigrants.

Tusk said EU leaders, at a meeting on Dec. 17, would have to overcome "substantial political differences" to meet some demands.