"The consultations I have led with all member states show the goodwill of the participants but it doesn`t change the fact that some parts of the British proposal seem unacceptable," Tusk told reporters.
Cameron is expected to use a Thursday dinner in Brussels to lay out his demands for reforms in European Union in four areas or "baskets" ahead of a British referendum over continued EU membership to be held by the end of 2017.
European Commission head Jean-Claude Juncker said he was convinced EU leaders would find a solution to "that highly complicated question" of benefits, which Cameron insists he must secure to ease concerns of euroskeptic British voters.
"We`ll enter the concrete and vital phase of negotiations with our British colleagues," Juncker told reporters. "The Commission is ready to look for other options than the single one proposed by the British prime minister."
Cameron has been touring European capitals to drum up support for demands that he says will keep the British public from supporting a so-called "Brexit," and make the UK the first nation to leave the 28-nation bloc.
Cameron has already won encouragement from German Chancellor Angela Merkel and other leaders who want to keep Britain in the bloc.
Impasse over welfare benefits
But it`s Cameron`s proposal to make immigrants from the other 27 EU states wait four years before claiming "in-work" benefits in Britain - payments to people in lower paid jobs to make work more attractive - has been roundly criticized, especially in eastern Europe, for its discriminatory nature.
In Britain, the voting public is split over membership of the European Union. The "out" campaign has gained some momentum by making the argument the bloc will lead to out-of-control immigration and Brussels` inability to tackle such crisis as a single entity.
But the former Conservative leader John Major has said Britain should not think of exiting the EU lest it leave the island nation in "splendid isolation" and that could lead to the break up of the United Kingdom. Support for the EU runs high in Scotland, which is ruled by the pro-independence Scottish National Party.
"I am skeptical of a great deal of European Union policy, but flirting with leaving at a moment when the whole world is coming together seems to me to be very dangerous," Major, the prime minister from 1990 to 1997, told BBC Radio.
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