David Cameron was not aware of Education Secretary Michael Gove's controversial plans to bring back O-levels until they were leaked this week, Nick Clegg has claimed.
The Deputy Prime Minister said neither he nor the Prime Minister were informed of the proposals - and stressed they would not become Government policy without Liberal Democrat backing.
Downing Street has refused to say whether Mr Gove shared his ideas with Mr Cameron before they were leaked, infuriating Liberal Democrats staunchly opposed to any return to what they see as "two tier" schooling. But Mr Clegg said: "This has not been subject to collective discussion in government. Neither myself nor the Prime Minister were aware of it."
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."
The Deputy Prime Minister said he looked forward to having "many more discussions" on raising standards in schools and ensuring that "all children can get ahead through hard work and aspiration".
A Number 10 spokeswoman said that the Prime Minister agreed with Mr Gove that "we do need to look again at the exam system". But she stopped short of confirming Mr Cameron's endorsement of the Education Secretary's specific plans, saying the Government's proposals had not yet been published.
The leaked plans include the most radical shake-up of the exams system for 30 years, 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 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".