The Deputy Prime Minister says planned changes to the exam system will be comprehensive but insists there will be no return to the two-tier system of qualifications, which pre-dated GCSEs.

The plans, to be unveiled by Nick Clegg and Education Secretary Michael Gove, will see the biggest overhaul of secondary school testing for a generation.

The changes, which apply only to England, are designed to introduce more academic rigour to exams for 15 and 16-year-olds amid concerns about falling standards and dumbing down since the introduction of GCSEs in the late 1980s.

It will mean an end to modular and rolling assessments and a stronger emphasis on the more traditional exam at the end of two years of study. There will also be a limit to the proportion of top grades that are awarded after years of ever-rising numbers of As and A*s.

Labour has not as yet indicated that it will oppose the plans, with Shadow education secretary Stephen Twigg saying: "Labour supports rigorous exams but only if they don't act as a cap on aspiration. Politicians should not set an artificial limit on the number of top grades, rather the best work should be rewarded.

"New exams should ensure that young people are prepared for the world of work and the jobs of the future. However, it is not clear how this new system will ensure a breadth of knowledge and skills and that pupils continue studying English and maths until age 18."