Announcing that GCSEs, A-levels and AS-level exams will not go ahead, Mr Williamson said: "Last year, all four nations of the United Kingdom found their arrangements for awarding grades did not deliver what they needed, with the impact felt painfully by students and their parents.
"Although exams are the fairest way we have of assessing what a student knows, the impact of this pandemic now means that it is not possible to have these exams this year.
"I can confirm that GCSEs, A-levels and AS-level exams will not go ahead this summer."
He also said that SATs will not be going ahead this year across England.
The grading of GCSE and A-level students in England became a fiasco last summer when end-of-year exams were cancelled amid school closures.
Thousands of A-level students had their results downgraded from school estimates by a controversial algorithm, before Ofqual - the exams regulator for England - announced a U-turn, allowing them to use teachers' predictions.
Mr Williamson acknowledged that exams are the "fairest way" of assessing what a student knows, but said the impact of the pandemic meant they could no longer go ahead.
He confirmed his daughter is one of the students who will now not be sitting her GCSE exams this year, and added he hoped today's statement had given students and parents "a clear sense of certainty and direction of where we are going".
"We will also be following this up with further detailed consultation by Ofqual that will be launched next week," he said.
On school closures he said that schools were "much better prepared than last March" to implement home-learning and that he expects schools to provide between three to five hours of teaching a day.
Boris Johnson said on Wednesday the lifting of restrictions would be a "gradual process" and not a "big bang" - and that schools would be "the very first things to reopen".
"That moment may come after the February half-term, although we should remain extremely cautious about the timetable ahead," the prime minister told MPs.
Education Select Committee chairman Robert Halfon earlier told Sky News the situation regarding schools was "a mess".
"I think now we have to move on and make sure we have an exam system that is a level playing field for students and fair to the disadvantaged," he said.
Downing Street said later that Mr Johnson continues to have confidence in Mr Williamson and believes he is the best person for the job.
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 added.
The other three nations are taking a similar approach, with First Minister Nicola Sturgeon imposing a lockdown on Scotland for the rest of January and the closure of schools until February.
Schools and colleges will be closed in Wales until at least 18 January and move to online learning, with GCSE and A-level exams already cancelled.
In Northern Ireland, pupils will learn remotely until the mid-term break and all scheduled GCSE and A-level exams have also been cancelled.
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