Exam board Pearson has apologised after announcing this year’s Sats results in England would be delayed by more than a week.

Pearson said the delay was caused by “technical issues”, pushing the publication of results from Tuesday 7 July to Thursday 16 July.

The National Association of Head Teachers (NAHT) said “something has gone badly wrong”, and that “schools must be given cast-iron assurances that the results they receive are reliable”.

Education Secretary Bridget Phillipson said the delay was “deeply frustrating” for schools, parents and pupils and that the government was working to resolve it.

Pearson said its delivery of GCSE, A-level and other qualification results would be unaffected.

NAHT general secretary Paul Whiteman called the Sats delay “completely unacceptable” and said it was “totally unfair” to land it on schools last-minute.

“Schools are held to an extremely high standard when it comes to test administration,” he said.

“Small errors can result in serious consequences for school leaders, and the same should be expected of Pearson and everyone responsible for test and exam results.”

It is the first time Pearson has delivered the Key Stage 2 tests, which are sat by Year 6 pupils in England, after a change in the contract run by the Standards and Testing Agency (STA).

Whiteman said he had previously been “reassured that everything was on track” after raising concerns with the STA about the change.

“Clearly, though, something has gone badly wrong and Pearson has questions to answer. We would expect serious consequences to follow,” he added.

In a statement confirming the delay, a spokesperson for Pearson apologised “unreservedly” for the disruption, and thanked its markers who had been affected by the technical problems.

One marker, who spoke to the BBC via Your Voice in June and asked not to be named, said internal deadlines had consistently been pushed back.

She said the technical issues meant she had stayed up until 01:00 some nights “just trying to keep up”.

She said she continued marking because students and parents were expecting to get their results in July and she did not want to see them disappointed.

She also raised concerns about the accuracy of the marking process.

“I marked one question that I remembered I had marked before, but the system had assigned the mark I gave to a different question,” she said.

“I reported this, but it raises concerns about whether errors like this could impact final results.”

Pearson said it had a plan to complete the remaining work, with teams “working around the clock to deliver the remaining pupil data”.

“Our priority is ensuring that every school and pupil receive complete and accurate results,” a spokesperson said.

Whiteman said it was “not the first time schools have faced delays”, adding that “once the current issues have been addressed it is vital that they are properly investigated to ensure it is the last”.

“Pupils, parents and schools have been badly let down,” he said.

“Given the fiasco around the marking and return of results, we once again question the whole purpose of these outdated tests.”

Additional reporting by Vanessa Clarke