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."