The head of the Government's school testing agency quit after being shown an advance copy of a damning report into last summer's Sats marking fiasco, The Times has learnt.

Ken Boston told the board of the Qualifications and Curriculum Authority (QCA) that he is leaving his job and “taking responsibility” over the marking delays, which affected a million children.

It was in the authority's interests to have new leadership, he said. The inquiry by Lord Sutherland of Houndwood into the affair will report tomorrow. However, his report arrived at the Schools Department on Thursday. Dr Boston was allowed to read it and made his decision on the basis of its contents. It is thought that he had little choice but to go and that the report would have made his position untenable.

However, the board has yet to accept his resignation, it is thought for contractual and legal reasons. It appears to have left itself the option of dismissing him depending on the findings of the report, which would change the nature of his departure terms. Dr Boston has faced controversy over the nature of his contract with the QCA, including an allegation yesterday that it contained a clause promising to pay his yacht club subscription.

The Conservatives are calling for ministers' role in the affair to be scrutinised.

The QCA and its testing arm, the National Assessment Agency (NAA), were responsible for the overall delivery of this year's tests, taken by children aged 11 and 14, and, at arm's length from ministers, appointed the private contractor ETS Europe to carry out the process.

Administrative problems meant that ETS failed to get the papers in English, maths and science marked in time.

As a result of the confusion, more than a million pupils' results were delayed and thousands of scripts went missing or were sent to the wrong place.

ETS is a big, non-profit-making American organisation with extensive experience in administering tests in 180 countries. It has a controversial history of alleged mistakes in its marking of American tests. The QCA severed its contract with ETS in August, and the firm was made to pay back £19.5 million plus £4.6 million for the cancellation of invoices and other charges. The five-year contract, which was in its first year, had been worth £156 million.

Michael Gove, the Shadow Children's Secretary, said: “It's vital that ministers are held accountable. We need to know how the Government ended up signing a multimillion-pound contract with a firm with such an appalling track record and what steps ministers took to ensure the effective delivery of this year's exams.”

Dr Boston said in a statement: “I have reflected since the summer on the delivery failure and on the difficulties associated with key-stage testing. In my 40-year career as a public servant in England and Australia I have always believed in public bodies and public officials taking responsibility when things go wrong.

“In the light of that reflection and that belief, and in view of the challenges facing the Qualifications and Curriculum Authority in the coming year, I believe it would be in the authority's interests to find new leadership.”

Dr Boston repeated his apology to pupils, teachers and parents for the failure to deliver the test results on time. The performance of ETS had been “quite unacceptable”.

John Dunford, the general secretary of the Association of School and College Leaders, said that Dr Boston had done a “magnificent job”. He said: “It is a tragedy for the education system that Ken Boston has felt it necessary to resign as chief executive of QCA.

“He has consistently spoken out about the shortcomings of the bloated testing and examinations system in England, describing it as a 'cottage industry'.”

Christine Blower, acting general secretary of the National Union of Teachers, said that Dr Boston had been a victim of the Government's policy on testing and assessment.

She added: “He was independent [and] set up a sensible curriculum and assessment policy. He has been the victim of an entirely unviable, unwieldy and unmanageable system.”

Ed Balls, the Schools Secretary, is expected to make a Commons statement on the affair tomorrow.

David Laws, the Liberal Democrats' education spokesman, said that he was “personally sorry” at the resignation.

He added: “I only wish that ministers would demonstrate a similar willingness to take responsibility.”

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