This comes ahead of a two-day summit of EU leaders, which begins on Thursday.
On Monday, EU Council president Donald Tusk said negotiations over the UK`s demands were at a "critical moment" and warned the risk of break-up "is real".
Mr Tusk is overseeing the negotiation, and was speaking after talks with the Romanian and French presidents as part of a tour of European countries ahead of the summit later in the week.
He said: "This is a critical moment. It is high time we started listening to each other`s arguments more than to our own. It is natural in negotiations that positions harden, as we get closer to crunch time.
"But the risk of break-up is real because this process is indeed very fragile. Handle with care. What is broken cannot be mended."
After talks between Mr Cameron and the French president, Francois Hollande, in Paris late on Monday, a Downing Street spokesman said they had held "constructive discussions" on the UK renegotiation.
"They agreed that we are making good progress... and that the draft text from the European Council provides a firm basis to reach agreement at this week`s summit," the spokesman said.
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