Education Secretary Michael Gove is 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.
The Liberal Democrats expressed fury 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.
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. A No 10 spokeswoman would not say whether Mr Cameron was even told of the O-level plan in advance, or if it would form part of a formal consultation on exam reform.
Defending his plans, Mr Gove 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."
The Lib Dems also 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. "We are very, very hostile to something that looks like it is going to return to the two-tier system of the past."
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."