The schools minister is being urged to pull this year's national spelling test after the actual paper was published in error on a government website.

Head teachers leader Russell Hobby wrote to Nick Gibb asking him to "free schools from the obligation to use this test".

The government admitted a "serious error" but simply asked anyone who had seen the test not to pass it on.

Half a million seven-year-olds are due to sit the same test in early May.

The mistake was discovered by a school running an official trial of the test, after teachers noticed some pupils recognised the words being tested.

They checked the Department for Education website, where they found it among the sample papers published to aid teachers and pupils.

It had been there since January.

'Serious error'

In his letter to Mr Gibb, Mr Hobby said: "We have no way of knowing how extensively it has been used by schools."

The test, which is being taken by infant children for the first time, is to be used to set the national standard for Key Stage 1 spelling for future years.

Mr Hobby added: "This is a serious error that undermines confidence in the administration of primary tests and also means that we can have little faith in any standard setting exercise that may emerge from the pre-test trials.

"It will not be possible to exclude schools from the trial to achieve a pure sample in any reliable way, and the schools themselves cannot know with certainty if parents have downloaded samples, or, indeed, if Year Two teachers have adapted the materials on their own initiative.

"It will be impossible to set a credible standard on this basis.

"I therefore ask that you free schools from the obligation to use this test."

Tests and assessments are taken by every seven-year-old and every 11-year-old in the country, and the results are used to hold schools to account.

However, these spelling and grammar test results are not used to rank the schools.

The teacher who spotted the error, Charlotte Smiles, said: "One of the children who was sitting the spelling test that we were giving them kept saying, 'I know this one, and this one'. He appeared to know what was coming next.

"Because of the way this child was behaving, I went and checked on the DfE website and I found this exact test published as a sample paper.

She said the DfE finally admitted the error after a number of telephone calls.

Officials have now removed the sample paper from the website and checked no other paper has been affected.

A DfE spokesman said it was "deeply regrettable it had happened" but stressed the results of this test were not collected nationally but kept within schools to inform teacher assessments..

"Fortunately, this is a Key Stage 1 test which is provided to schools to support teacher assessment judgements.

"We ask if anyone has seen the material that they do not share it further so the test remains helpful for those teachers who have to yet use it with their pupils."

National Union of Teachers general secretary Christine Blower said primary assessment was plainly in a mess.

Shadow education secretary Lucy Powell said the incompetence of ministers was "breathtaking".

"To publish upcoming test papers online, before children have had a chance to sit them, renders the whole process null and void," she added.