Gavin Williamson ignored advice to hold socially-distanced tests rather than scrapping exams during the pandemic, the head of the regulator has said.
Ofqual chairman Roger Taylor said it wanted to delay exams or offer 'teacher certificates' - but the Education Secretary pushed for exams to be scrapped.
Mr Taylor told MPs it had been a "fundamental mistake" to think awarding grades using a controversial computer algorithm would "ever be acceptable" to the public.
Ofqual's chief Sally Collier stood down last week following the A-Level fiasco, where thousands of students had their marks initially downgraded before the Government was forced to make a u-turn.
Exams across the UK were scrapped due to coronavirus, with students instead being given predicted grades by their teachers.
But the scheme was plunged into chaos when 40% of grades were reduced by Ofqual's computer model - forcing Mr Williamson to make a humiliating u-turn days later.
Following the claims by Ofqual, Labour accused Mr Williamson of pointing the finger at officials and told him to "to take responsibility for his own incompetence".
Mr Taylor told the Commons Education Committee that Ofqual advice to Mr Williamson was to try and hold the summer exams in a "socially-distanced manner", with the second option being to delay exams.
He said: "The third option - if neither of these were acceptable - would be to have to try and look at some form of calculated grade.
"We did also look at whether that might be a teacher certificate rather than attempting to replicate exam grades. That was our advice to ministers.
"It was the Secretary of State who then subsequently took the decision and announced without further consultation with Ofqual that exams were to be cancelled and the system of calculated grades were to be implemented."
He added that opting for the third option of calculated grades was the "worst-case scenario".
Ofqual officials said they repeatedly raised the risks of the model with Government throughout the pandemic.
Julie Swan, Ofqual's executive director for general qualifications, said a paper on May 1 "highlighted the risk of widespread dissatisfaction" over grades.
Tory MP Robert Halfon, chairman of the committee, said Ofqual's defence of their performance was it's "not me guv".
He said: "Perhaps some of the wild anomalies that sixth form colleges saw surely would have become apparent much sooner.
"In essence, what I am asking is, should you have done your own mock exam in terms of the algorithm?"
Dr Michelle Meadows, executive director for strategy, risk and research at Ofqual told MPs: "We tested the model thoroughly.
"We were confident that the model that we chose was the most accurate overall and the most accurate for those different groups of students."
She said the risks with the model were raised with the Department for Education (DfE) "throughout the process".
Shadow Education Secretary Kate Green said their evidence "raised serious questions about Gavin Williamson’s role in this summer’s exam fiasco".
"Gavin Williamson has repeatedly tried to blame Ofqual and officials for the crisis over exams," she said.
"It is now clear he was responsible."
"Williamson must urgently come to the Commons to offer an explanation and to take responsibility for his own incompetence."
A Department for Education spokesperson said: “As we’ve consistently said, the Government never wanted to cancel exams because they are the best and fairest form of assessment.
“We listened to views from a range of parties, including Ofqual, and given the public health requirements at the time, made what was a very difficult decision on the basis that it was a necessary step to fight the spread of coronavirus.
“We welcome the work of the Education Select Committee and look forward to engaging with it while working closely with Ofqual to ensure fairness for students both this year and in years to come.”
