Britain's Cameron appeals to EU reform doubters at home, abroad

Cameron, under pressure after overseeing what critics see as a lackluster start to the renegotiation of Britain's ties with the European Union, used a speech to offer the most detail yet on four areas where he wants change.

He outlined the strong economic and foreign policy role Britain plays in the EU but said he did not doubt that Britain could survive if its people voted to leave the EU in a referendum by the end of 2017.

Cameron asked for understanding from Eurosceptics at home and from European partners, some of whom have become frustrated with London's demands for change.

"There will be those who say, here and elsewhere in the EU, that we are embarked on 'Mission Impossible'," Cameron told diplomats, reporters and business leaders at the Chatham House think tank in London.

"I do not believe so for a minute ... The European Union has a record of solving intractable problems. It can solve this one too. Let us therefore resolve to do so, because the prize is a big one."

Cameron also sent a letter to Donald Tusk, president of the European Council, setting out his four main objectives to launch what he described as the renegotiation's "formal phase" before a December summit when EU leaders will hammer out the details of Britain's new terms.

Tusk said in Brussels that talks would start next week.

The renegotiation could be difficult, with the question of benefits to EU migrants likely to cause the most trouble.

The EU executive said it saw demands for new limits on the rights of EU citizens working in other member states as "highly problematic".

In his speech, Cameron spelt out what he wanted from Brussels.