GCSE and A-level students will receive grades based on teacher assessments after exams were cancelled in a bid to slow down the spread of coronavirus.
The calculated grades, which will also take into account students' previous achievements, should be awarded to pupils in England by the end of July, the government said.
Education Secretary Gavin Williamson said these are "extraordinary times" and that exam boards would be working closely with teachers to implement the new approach.
The exam regulator, Ofqual, and exam boards will work with teachers to provide grades to students whose exams have been cancelled this summer.
University representatives said they expect universities to be flexible and do all they can to support students and ensure they can progress to higher education.
This year's summer exam series, including A-levels, GCSEs and other qualifications, have been cancelled as the coronavirus pandemic continues to escalate in Britain.
The government said its priority is to ensure affected students can move on as planned to the next stage of their lives - such as starting university, six form courses or employment.
Students will have the option to sit an exam as soon as schools reopen and can appeal if they are not satisfied that their calculated grade reflects their performance.
They can also choose to sit exams in summer 2021.
The new guidance says exam boards will be asking teachers to submit judgements about the grades they think their students would have got if exams had gone ahead.
Teachers will have to take into account "a range of evidence and data" such as mock exam results and other school work.
Combined with information from "other relevant data" such as pupils' previous attainment, the calculated grades will be "a best assessment" of the work that students have put in, the government said.
A-level and GCSE grades are usually published in mid-August.
Mr Williamson said: "Cancelling exams is something no education secretary would ever want to do, however these are extraordinary times and this measure is a vital but unprecedented step in the country's efforts to stop the spread of coronavirus.
"My priority now is to ensure no young person faces a barrier when it comes to moving on to the next stage of their lives - whether that's further or higher education, an apprenticeship or a job."
He continued: "I have asked exam boards to work closely with the teachers who know their pupils best to ensure their hard work and dedication is rewarded and fairly recognised."
The department added that the distribution of grades will follow a similar pattern to previous years so affected students do not face a "systemic disadvantage".