Michael Gove has appeared to soften his position on radical reforms that would see the return of O-levels and CSEs after an outcry over the proposals.
The Education Secretary said he would like to see all students sit O-level-style exams at some point in their school career.
It could mean that proposals for less able pupils to sit easier CSE-style exams would be ditched.
Mr Gove had "ruled out as clearly as I can a two-tier system" saying he wants to move towards a one-tier set of high quality qualifications.
Under plans leaked last week, GCSEs would be scrapped with pupils sitting "explicitly harder" O-level style 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.
But the move led to uproar from teaching unions and education experts as well as exposing deep divisions within the coalition.
Lib Dem sources suggested they were "very, very hostile" to something that would create a two-tier system while deputy prime minister Nick Clegg said he and prime minister David Cameron knew nothing about it.
In an opposition day debate in the Commons, Mr Gove appeared to suggest that pupils may be able to do O-levels at different stages of their school career.
He said: "There's nothing we wanted to do which is a step backwards but everything that we want to do which is a step towards the high-class qualifications that other countries have.
"I have ruled out, I think as clearly as I possibly can, any two-tier system. I have said we want to move to one-tier and a set of high qualifications."
In a speech earlier, Mr Gove said he wants to raise the bar on qualifications over the next decade, to bring England in line with top performing countries like Singapore.
"I want us to ensure that in the next 10 years at least 80% of our young people are on course to securing good passes in properly testing exams in maths, English and science - more rigorous than those our children sit now," he said.
"This goal, while explicitly ambitious, is also entirely achievable. In Singapore the exams designed for 16-year-olds embody all those virtues and are taken successfully by 80% - and rising - of the population."
He later added that he believes it is important to "personalise" learning.
"Ultimately I think that 80% is just a staging post," Mr Gove said.
"What I'd like to see is the sort of society in which every child, unless they face the most severe learning difficulties, has the opportunity at the age of 16 to make their own choices."
Mr Gove added: "I think one of the most debilitating things in our entire education system has been the idea that you make judgments early in a child's career about what they're fit for.
"And I think one of the things we need to do is to make sure that hopefully by the age of 16, but if not at 16 then at 17 or 18, that a child is equipped with the level of qualifications to make those choices themselves."
His comments are likely to been seen as a shift from last week's proposals, which suggested that less able pupils will sit simpler examinations similar to the old CSEs.
The education secretary also denied there is a rift in the coalition over his GCSE plans.
He insisted that deputy prime minister Nick Clegg backed his reforms and said he had been "encouraged by the degree of support" from the Lib Dems.
Under the current system, some children are doing equivalent qualifications that are not good enough while others are sitting "foundation level" GCSEs in which the highest grade they can achieve is a C, Mr Gove told the Spectator conference in central London.
He told the Commons that GCSE foundation papers were "explicitly designed to limit a student's success".
Under the new system, children who fail to take the O-level style qualification at 16 would be able to take the exams at 17 or 18 as they progressed, he said.
He said: "We want to ensure that we develop qualifications which are not only without the current tiers that set a cap on aspiration, but qualifications which can be taken at different points in a child's career."
Asked by shadow education secretary Stephen Twigg whether tiering would no longer be needed "because 20%-25% of children" would be sitting the new CSE instead of the O-level, the education secretary told him: "What we want to do is ensure that more and more of our children are doing better and better."
He added: "I think more children can succeed if we make our exams more demanding because we have a higher degree of aspiration and ambition for all our children."
Speaking later, Mr Twigg said: "The 'omni-shambles' has become an O-level-shambles.
"Labour was the first to oppose the planned return to the two-tier system, so this apparent U-turn will be a welcome relief to many families.
"Following the outcry over his proposals, Michael Gove appears to have been forced into changing his mind, but parents, pupils and teachers will remain totally confused as to what exams will be taken in 2015."