The leak raises serious questions about the security surrounding the tests which have been the subject of a series of cheating allegations.
Chris Woodhead, chief inspector of schools, argued that national tests, taken by two million children each year, were vulnerable to cheating.
When inspectors investigated the leak in the London borough of Wandsworth they found that one head teacher had opened the test papers in advance in defiance of Government rules.
Government exam advisers, who held an urgent inquiry, said they were confident no school in Wandsworth had cheated after being told that the comprehension was a poem about spiders. The borough's results will be checked carefully.
Information about the comprehension was first passed to a group of teachers by one of Wandsworth's literacy consultants at a training session on how to administer the tests.
When they discovered the leak, inspectors in the borough phoned round some of the schools that had not been at the session to give them the same details, in order, it is believed, to ensure that there would be a level playing field.
Yesterday, officials from Wandsworth and the Qualifications and Curriculum Authority (QCA) which supervises national tests visited all the borough's schools to question heads about whether they had coached children as a result of the leaks. About 30 of the 60 schools said they knew about the spiders.Reuse content