VALLETTA (AFP) - Top EU official Donald Tusk warned Thursday it would be "very, very tough" to reach a deal on reforms Prime Minister David Cameron has asked for to help keep Britain in the bloc.
Cameron outlined four key demands in a letter to Tusk, the president of the European Council, earlier this week.
Responding publicly for the first time, Tusk said Thursday: "For sure the requests are tough and this is why the letter was so interesting for me."
Tusk said it would be very difficult to reach an agreement and said there was "no sure guarantee" a deal could be done by December, when EU leaders are due to review Britain's proposals at a summit in Brussels.
"For sure it is a very, very tough game," Tusk, a former Polish prime minister, added.
Tusk said detailed negotiations would begin next week with the other 27 member states invited to "a kind of confessional" to give their views on the changes London is seeking before a planned referendum on whether to leave, which Cameron has pledged to hold by 2017.
The Conservative premier has refused to rule out the possibility of campaigning for 'Brexit' - a British exit from the EU - if his reform proposals are not approved.
The most controversial aspect of Cameron's proposals is a request for the right to restrict welfare and in-work benefits for newly arrived migrants from other European Union states.
Eastern European countries with many citizens working in Britain regard the proposal as discriminatory and the European Commission has described it as "extremely problematic".