The results of national curriculum tests taken by 1.2 million 11 and 14-year-olds in England are to be delivered to schools a week late amid concerns over the accuracy of marking.

Ed Balls, the Schools Secretary, announced that more time was needed to complete the marking of papers and to “resolve technical issues”.

The new qualifications regulator Ofqual responded immediately by announcing an inquiry into the delay, which it described as “unacceptable”.

The hold-up means that parents and children hoping to receive the results before the end of term may be disappointed. Results for Key Stage 2 are now expected to be received by schools a week late on July 15, and for Key Stage 3 by the end of next week.

Teaching unions and opposition MPs said that the failure raised serious questions about the wisdom of awarding the contract for marking the papers to a private contractor for the first time this year.

The National Assessment Agency, the government body responsible for the delivery of the tests, described the problem as a “serious failure” on the part of the American-owned contractor ETS Europe. It attributed the problem to a series of additional quality assurance checks introduced by ETS.

ETS took over marking of the 9.5 million national curriculum Key Stage 2 and 3 papers on a five-year contract worth £150 million. Yesterday it admitted that its examiners would still be working over the weekend to complete the backlog of marking.

It insisted that there were no questions about the accuracy of its marking, but accepted responsiblity for a number of delays. In some instances contracts with individual markers had not been drawn up in time. In other cases there had been delays in delivering exam scripts. There had also been technical problems with a new system for recording marks online.

The company said in a statement: “We will report the vast majority of the test results one week late, on July 15, with a small number of Key Stage 3 results to follow as soon as possible.”

Michael Gove, the Shadow Schools Secretary, said that the company had a poor track record, which the Government had overlooked when the contract was awarded. “The Government awarded the contract for exams to a company with a record of failure which we exposed. Now at the eleventh hour the results have been delayed and question marks hang over their credibility,” he said.

“Given that the marking has been so botched, parents will now be wondering how they can be sure that the results received by their child are accurate,” he added.

Chris Keates, of the NASUWT teaching union, questioned the wisdom of contracting-out marking to a private sector company. “It is clear that this key responsibility of the State should not be handed over to private sector profit-making organisations. Taxpayers should ask if this is the best way to spend public money,” she said.

John Dunford, general secretary of the Association of School and College Leaders, said the problems provided further evidence that the high-stakes testing regime in England had become too extensive and unmanageable.

More than 90 examiners have contacted The Times Educational Supplement to complain about problems with administration, and hundreds of criticisms from markers about training and quality assurance have been posted on the paper’s website.

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