Amid Lib Dem fury at Michael Gove's leaded plans to scrap GCSEs and replace with them with something resembling the old O-Level model, Nick Clegg spoke out against the old system that cast some children "on the scrapheap".
Clegg expressed strong opposition to the proposals, speaking from Brazil where he is attending a global environmental summit.
He said: "This was self-evidently not policy that has either been discussed or agreed within the coalition Government.
"Mr Gove is entirely entitled to come up with proposals and then, if he wants to, we can then discuss them within the government."
The Liberal Democrats earlier expressed outrage at the leaked plans, insisting they would not be allowed to go ahead. A senior source said party leader Nick Clegg and his colleagues only learned of the idea when they read the Daily Mail.
Questioned about his view of the proposed return of O-Levels during a television interview in Rio de Janeiro, Clegg said: "I'm not in favour of anything that would lead to a two-tier system where children at quite a young age are somehow cast on a scrapheap.
"What you want is an exam system which is fit for the future, doesn't turn the clock back to the past and rewards hard work and effort for the many, many children in our school system who work hard and want to get ahead, so it works for the many and not just for some and not just for the few."
A senior Lib Dem MP told The Huffington Post UK that the plans seemed to be based on "nostalgia" and a "desire to replicate the public school system".
"I remember as one of the last pupils to go through the old O-level system that those kids taking CSEs were seen as less important," the MP told us.
Education Secretary Michael Gove is now facing a major backlash over his controversial plans to bring back O-levels.
As concerns deepened in education circles that the move risks branding teenagers as failures, Downing Street pointedly declined to say whether Prime Minister David Cameron approves of Mr Gove's proposals.
Documents obtained by the newspaper describe 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 will write off large swathes of the population.
The Lib Dems dismissed the plan as a return to the "two-tier" system of the 1950s, saying no one outside of Tory Cabinet minister Mr Gove's office appeared to have known it was being considered.
"Nick Clegg and the Lib Dems do not want to return to the divisions we saw in the 1950s," the source said.
The party would not accept a policy which would leave "a large number of children behind at a relatively young age".
They said: "We are very, very hostile to something that looks like it is going to return to the two-tier system of the past."
Earlier today Mr Gove was hauled before the Commons to answer an urgent question on the shake-up.
Gove confirmed the government thought the examination system was not fit for purpose and was contributing to declining education standards compared with other countries.
During his statement to MPs there were very few Lib Dems in the chamber, prompting speculation that the party wasn't in favour of the root-and-branch reforms to GCSEs.
Defending his plans, the minister said: "The truth is we have a two-tier system in education in this country. Some of the most impressive schools have already left GCSEs behind and opted for more rigorous qualifications like IGCSEs.
"While there were undoubtedly improvements in our schools and by our teachers over the course of the last 20 years, those improvements were not sufficient to ensure that we kept pace with other jurisdictions."
Shadow schools minister Kevin Brennan claimed that Mr Gove and his ministers are in favour of "going back to the future".
He said: "They want to bring back a two-tier exam system designed in the 1950s which will separate children and close off opportunity."
Conservative MP Graham Stuart, chairman of the Education Select Committee, said he was sceptical about the proposals and that they came "out of the blue".
He told BBC Radio 4's The World At One: "It does raise a lot of questions. The government says it has two main educational goals: to raise standards for all and to close the gap between rich and poor.
"How exactly will a move back to traditional O-levels and some as-yet-unspecified other form of examination for lower performing pupils help close the gap between rich and poor? How will it increase social mobility?"
Lib Dem MP and member of the Education select committee David Ward said Gove was "dangerously fixated on a fantasy."
In a statement on his website Ward accused Gove of wanting "to return Britain to a 'golden age' of education that never really existed. Following his previous announcements on rote learning, I wouldn't be surprised if chalk boards and compulsory fountain pens are next in the pipeline.
"He needs to realise that the world has moved on. His proposal for a two tier system risks undermining decades of progress on fairness and social inclusion in schools. And the last thing our education system needs now is yet more disruptive upheaval, distracting teachers' attention away from where it needs to be - on driving up standards for every child.
"Rather than taking us back to an age when most children started their adult life saddled with qualifications that were seen as second rate, the Education Secretary would be better off focussing his attention on giving teachers the freedom and resources they need to stretch all pupils and creating a culture of high expectations in every school."
Brian Lightman, general secretary of the Association of School and College Leaders, said the proposals "risk labelling teenagers as failures".
Russell Hobby, general secretary of the National Association of Head Teachers, warned: "The return to a two-tier system, CSEs and O-levels implicitly labels some children as less capable."
Under the proposals, pupils would begin studying "explicitly harder" exams in traditional academic subjects such as English, maths, history, modern languages and the sciences from 2014, with exams taken for the first time in 2016.
Papers would be set by a single exam board to provide a "gold standard" test across England, the documents say.
It would mean that schoolchildren currently in Year 8, aged 12 and 13, would be the last to take GCSEs.
Less able pupils will sit simpler examinations similar to the old CSEs. They will include simpler tests in English and maths, to provide them with "worthwhile" qualifications.