Nick Clegg is to join forces with Michael Gove on Monday to launch the exam system that will replace GCSEs after the next general election in 2015.

The joint appearance will mark something of a rapprochement between the two cabinet ministers who clashed in June when Gove indicated he would abolish GCSEs in favour of a two-tier system based on the old O-levels.

In a statement rushed out at the Rio+20 summit, which he was attending when the Gove reforms were revealed by the Daily Mail, Clegg rejected a two-tier system out of hand on the basis that it would cast children on to a "scrapheap".

The deputy prime minister, who has been involved in intense discussions with Gove and ensured that his lieutenant David Laws was appointed a schools minister in the recent reshuffle, has forced Gove to make two major concessions:

• There will be no two-tier system and the new exams will be sat by most pupils. Gove originally proposed a system similar to the old two-tier system in which brighter students sat O-levels and less able ones sat CSE exams.

• Pupils will start to be taught for the new exams in autumn 2015, a year later than expected. The delay is to allow schools to make proper preparations for the wide-ranging change. This means Labour could scrap the new system if it wins the election in the early summer of 2015.

The changes will lead to an overhaul of the module system as course work is replaced by exams. Fewer students will achieve the higher grades as the government seeks to raise academic standards. It is expected that around 10% of pupils will be awarded a grade 1. Under the current system around a third of pupils are awarded A or A* grades. There will also be only one exam board. Ministers believe exam boards have in recent years competed for business by making it easier for pupils to obtain higher grades.

The announcement by Gove and Clegg follows the row when students were marked down to curb grade inflation in line with the wishes, if not the instructions, of ministers. Gove has admitted that students were treated unfairly after the boundaries between C and D in English literature and English language were raised between January and June. This resulted in the awarding of fewer C grades. But Gove said it would be a "scandal" if he intervened.

Clegg has been careful not to criticise Gove in recent weeks because they have been in agreement, even at the height of their row over the summer, that schools in England need a more rigorous exams system. In his statement from Rio in June, the deputy prime minister said: "We need to make sure that we constantly improve the exam system so that it's rigorous and that it's stretching."

The joint appearance by the deputy prime minister and education secretary was originally scheduled to take place on Tuesday. But this has been brought forward to Monday after details of the new exams were leaked to the Mail on Sunday.

Labour criticised the government for leaking details of the reform, but declined to reject them out of hand. Stephen Twigg, the shadow education secretary, said: "It is inappropriate for an overhaul of GCSEs to be leaked while young people taking English GCSEs this year have been treated so unfairly, and are still in limbo. 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. There has been no consultation on these plans, rather they have been drawn up in secret and leaked to select media outlets."