GCSE and A-level grades are likely to be assessed using 'mini exams', according to the education secretary and England’s exam regulator Ofqual.
In a letter to the exams regulator, Williamson said this would help teachers to decide "deserved grades".
Williamson told Simon Lebus, Ofqual’s interim chief regulator, that the Department for Education and Ofqual would hold a joint consultation lasting two weeks to determine the final process to be used in awarding grades.
The letter, published on Wednesday morning, came as Mr Williamson appeared before the education select committee to answer questions on the impact of Covid-19 on education.
"A breadth of evidence should inform teachers' judgements, and the provision of training and guidance will support teachers to reach their assessment of a student's deserved grade," Williamson said in the letter.
Earlier this month, Mr Williamson's pledged not to use an algorithm to determine grades. This comes after thousands of A level students had their results downgraded from school estimates last summer.
Williamson said: “We have agreed that we will not use an algorithm to set or automatically standardise anyone’s grade.”
He added: "Schools and colleges should undertake quality assurance of their teachers' assessments and provide reassurance to the exam boards. We should provide training and guidance to support that, and there should also be external checks in place to support fairness and consistency between different institutions and to avoid schools and colleges proposing anomalous grades."
In his response, Lebus warned that this year’s grades would be less reliable than if exams had been held as usual.
“It is important that the consultation makes clear to all, especially those who rely on the results to make selection decisions, that overall outcomes this year will likely look different from 2020 and previous years,” Lebus said.
Geoff Barton, head of the Association of School and College Leaders, said the letter set out "broad and sensible parameters" for assessing GCSEs and A-levels after exams were cancelled.
"But, as ever, the devil will be in the detail of how this is turned into reality," Mr Barton said.
Dr Mary Bousted, National Education Union joint general secretary, said: "Had the government listened to the NEU and put in place a contingency plan sooner we would be in a better position now to make sure grades could be awarded reliably and without creating severe workload issues for education staff and students.
She said the union would continue to work with the Dfe and Ofqual, but they needed to see the full details of the plans as soon as possible to ensure grades are fair and the process is manageable for staff.
Williamson confirmed that exams for vocational and technical qualifications scheduled for February and March would not go ahead, while qualifications such as BTECs and Cambridge Nationals taking place this summer would be assessed in similar fashion to A levels.
Taking questions from MPs on the education select committee, Mr Williamson said he wanted to see schools re-opening at the earliest opportunity and that he would "never apologise for being the biggest champion for keeping schools open".
Meanwhile, new figures from the DfE showed that most primary schools in England had more than 20% of their pupils attending in-person this week – adding to evidence of much higher attendance during the latest lockdown compared with last year.