The move is designed to avoid the chaos seen last year when an algorithm downgraded the results of tens of thousands of pupils.
A resulting outcry forced ministers to perform a high-profile U-turn, but not before the computer programme had created misery for many.
Labour accused Gavin Williamson, the education secretary, of “incompetence” over his handling of schools, which were closed across England on Monday night just hours after Boris Johnson declared them safe.
Just last week Mr Williamson himself said: “I’m confident that we won’t be moving into a national lockdown situation because the tiering structure is the right place to be.”
Announcing the new plans, the education secretary told MPs he was unveiling the “contingency plans” he had prepared.
He vowed not to “let schools be closed for a moment longer than they need to be”.
But he conceded exams could not go ahead as planned this year. A form of teacher-assessed grades would ensure marks are awarded “fairly and consistently”, he said.
He told the Commons: “While the details will need to be fine-tuned in consultation with [the regulator] Ofqual, the exam boards and teaching representative organisations, I can confirm now that I wish to use a form of teacher-assessed grades, with training and support provided, to ensure these are awarded fairly and consistently across the country.”
In August, Mr Williamson bowed to intense pressure from pupils, parents and even other ministers to upgrade thousands of A-level and GCSE results.
A system, introduced at short notice after the coronavirus pandemic cancelled last year’s exams, used an algorithm which downgraded four in 10 teacher-assessed marks.
In the end, pupils had their original grades restored, but it was too late for many to take up their original first choice of university course last year, even after ministers removed a cap on the number of places.
Mr Williamson also told MPs that this month’s exams for technical qualifications could go ahead but added that no college should feel pressured to offer them.
But he said the government would not proceed with primary schools SATs exams this year.
He also confirmed that free school meals would be available while schools are closed and said there would be “extra funding” to pay for them.
The National Education Union criticised Mr Williamson for not announcing the move to teacher-assessed grades earlier.
Mary Bousted, NEU joint general secretary, said: “Gavin Williamson said in parliament today that he and Ofqual had prepared a contingency for teacher-assessed grades to award qualifications in summer 2021 but, if this is the case, why did he keep it from the sector? The NEU alongside other unions had called for structures to enable such a back-up option to exams in October. Had these structures been put in place then we would be in a much better position now to make it happen.
“Instead, there is a danger that implementing such a process fairly and consistently nationally at this late stage will lead to further extreme stress and workload for education staff, students and parents. This stress could have been avoided had government not been so obsessed and blinkered by their pursuit of exams in the face of the obvious prospect that they may not be fair or possible.”
Geoff Barton, from the Association of School and College Leaders, said: “It is frustrating that there is not an off-the-shelf plan B ready to go. We have repeatedly called on the government and the regulator to prepare such a plan in the event of exams being cancelled, and have repeatedly offered to work with them in doing so.”
“However, ministers have been so busy insisting that exams will take place that they have failed to ensure that there is a contingency system which can be immediately rolled out.”
The union’s general secretary added: “Ofqual now faces a race against time to come up with the ‘fine-tuning’ of a credible alternative to exams.”
Natalie Perera, from the Education Policy Institute (EPI), welcomed the decision to scrap GCSE and A-level exams this summer, but said Wednesday’s statement on the decision was “notably short on detail”.
“The government and Ofqual now need to act with some speed, so that students and schools can quickly adapt to the new expectations,” the EPI chief executive said.
On Wednesday, the prime minister’s press secretary, Allegra Stratton, told reporters Mr Williamson had produced a “full and comprehensive” package of measures for children who will be educated at home.
“It’s a huge brief and the prime minister believes the education secretary is doing it to his utmost ability,” Ms Stratton said.
One student, Mana Ali, told The Independent she wanted an option to be able to sit exams in 2021. She had been planning to resit her A-levels last year before they were cancelled – and had been revising to take them this year instead.
“I am resitting to prove what I am capable of hence why I want to sit my exams and not rely on centre-assessed grades,” she said.