Downing Street has denied claims that the Prime Minister was warned by his top civil servant that a planned four-year ban on benefits for EU migrants could be illegal.
The requirement for migrants to pay into the system through taxes for four years before receiving in-work benefits is a key demand in Mr Cameron's renegotiation of Britain's EU membership leading up to the in/out referendum promised by the end of 2017.
But the BBC reported that Cabinet Secretary Sir Jeremy Heywood had advised the PM that any ban would have to be restricted to six months or less to avoid being ruled discriminatory against non-British migrants.
A Downing Street spokesman said that it was "not true" that Sir Jeremy had provided this advice.
Mr Cameron is expected to set out details of his renegotiation demands in a letter to European Council president Donald Tusk next week, ahead of a full discussion with leaders of the other 27 EU member states in Brussels in December.
Number 10 insisted that the PM has not backed down on the demand for a four-year ban, first set out in a speech in November 2014.
Mr Cameron said then that he regarded the proposed four-year threshold as "plain, decent, reasonable, fair common sense", adding: "If Europe says no to that as the basis of our country staying in this organisation, then people will want a pretty good explanation why and, frankly, so will I."
And the Downing Street spokesman said the PM's view on the importance of the issue was unchanged.
"The Prime Minister's position has not changed from what he set out in his speech last year around the four years," the spokesman told a regular Westminster media briefing. "He set out in that speech that these were the things he was looking to renegotiate - the four years being one of them. That hasn't changed. That remains the same."
A Downing Street spokesman later said: "The story about the advice the cabinet secretary gave to the PM is not true. The PM remains committed to the proposal that EU migrants should live in the UK for four years before they can claim in-work benefits. That's what he is negotiating."