Downing Street has contradicted Nick Clegg by confirming that David Cameron had known about controversial plans to scrap GCSEs before they were leaked.
The Liberal Democrats reacted furiously this week when it emerged that Tory Education Secretary Michael Gove had drawn up the proposals.
The Deputy Prime Minister told BBC Radio 4's The World At One earlier: "This has not been subject to collective discussion in government. Neither myself nor the Prime Minister were aware of it. That's self-evidently the case."
Speaking from Rio, where he is attending an environmental summit, he indicated that Mr Gove stood no chance of making his reform plans government policy without the support of his Lib Dem colleagues. "If Michael Gove wants to turn some of his ideas into government policy he's entirely entitled to put that forward for wider discussion," he said.
Asked whether the proposals would not go ahead with his support, Mr Clegg added: "By definition, in a government, if you have collective agreement, and particularly in a coalition, it requires support from all sides."
However, Number 10 indicated that Mr Cameron had discussed the plans with Mr Gove previously - although he had not expected them to become public.
A senior aide said the premier agreed that major reform was needed in the exam system, but stopped short of endorsing Mr Gove's specific plans.
The documents leaked to the Daily Mail suggest replacing GCSEs with O-levels in traditional academic subjects such as English, maths, the humanities and science. The changes would also see less able pupils taking simpler qualifications, similar to old-style CSEs, and the national curriculum for secondary schools abolished.
School leaders warned that the "bombshell" move would write off large swathes of the population.
Mr Clegg said on Thursday that he was against "anything that would lead to a two-tier system where children at quite a young age are somehow cast on a scrapheap".