More time is needed to complete the marking of papers and "resolve technical issues," said Mr Balls, MP for Normanton.

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 the week of the 15th.

An independent inquiry into the reason for the delay has been launched and will report to the new qualifications regulator Ofqual.

Children aged 11 and 14 in schools across England took Sats in May in English, maths and science, which are regarded as important updates on individual progress and feed into education league tables.

Pupils, teachers and parents had been expecting the results next Tuesday.

But Mr Balls, pictured, revealed that just 90 per cent of Key Stage 2 marks and levels are ready, while the Key Stage 3 position is "less close to being complete".

In order to ensure an orderly release of results, it has been decided to hold the Key Stage 2 results back to July 15, when "the vast majority of marking" will be finished.

But he said that Key Stage 3 marking is expected to take longer, and results which are available by the end of that week will be released then to ensure that as many schools as possible receive them before the summer holidays.

In a letter to Huddersfield MP Barry Sheerman, chairman of the House of Commons Children, Schools and Families Committee, Mr Balls said he had been told of the delay by the Qualifications and Curriculum Authority on behalf of the National Assessment Agency (NAA).

The NAA described the delay as "a serious failure" on the part of its new marking contractor, the private firm ETS Europe.