But he said he was confident of getting what he wanted from reform talks.
Anti-EU campaigners say the talks are a "gimmick" - and the European Commission said the UK`s benefits proposals could break free movement laws.
Mr Cameron has formally set out his demands in a letter to the president of the European Council, saying four objectives lie at the heart of the UK`s renegotiations:
- Protection of the single market for Britain and other non-euro countries
- Boosting competitiveness by setting a target for the reduction of the "burden" of red tape
- Exempting Britain from "ever-closer union" and bolstering national parliaments
- Restricting EU migrants` access to in-work benefits such as tax credits
Mr Cameron hit back at claims by former Tory chancellor Lord Lawson that the four goals were "disappointingly unambitious", saying they reflected what the British people wanted and would be "good for Britain and good for the European Union".
"It is mission possible and it is going to take a lot of hard work to get there," said the prime minister.
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