Cameron sought to defuse fellow EU leaders' frustration that over a lack of detail in his demand for new membership terms by promising on Thursday to send them his wish-list in writing in early November.
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A spokesperson for his office told reporters that the letter to Council President Donald Tusk would contain "a bit more detail" on the changes Britain wants, paving the way for detailed discussions with member states.
When asked to clarify whether the letter would have a bit more, or a lot more, detail, the spokesperson said reporters would have to wait for the letter.
Tusk on Friday welcomed Cameron's decision to lay out his thinking in writing, saying real negotiations could then begin.
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Cameron, who opposes any further transfer of sovereignty to the EU and says British voters are unhappy with the current settlement, wants to renegotiate relations with the bloc it joined in 1973 ahead of a referendum on membership before the end of 2017.
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