The teachers see the test papers because they are also markers. But the revelation has prompted heads to call for safeguards to protect the tests’ integrity.

There are about 10,000 markers nationally, and most are full-time teachers. Forty per cent of KS 2 markers and 65 per cent of KS 3 markers are classroom teachers. It is thought likely that many of these will be preparing pupils for the same tests that they mark.

Getting papers early leaves these staff with a conflict of interest and there are fears the fairness of the tests may be undermined.

John Dunford, chief executive of the Secondary Heads Association, said: “This could put teachers in the position where they might unconsciously, as well as consciously in some cases, help their students.

“Any kind of national testing must have a level playing field and be seen to have a level playing field.

“It is good exampractice for the absolute minimum of people to see the papers beforehand,

as is the case with GCSEs and A-levels.”

Chris Davis, spokesman for the National Primary Heads Association, said: “I cannot see the need, for the sake of 10 days, why teachers have to be put in this position.

“There should be a rule that markers should not be teaching the level of class that they are marking.”

A QCA spokesman said: “We want to use teachers to mark the tests. They are the people who teach the children.

“Markers only see the material 10 days before the test which is not long enough to affect their preparation of pupils - intentionally or unintentionally.

“All the material they receive contains confidentiality warnings. From our follow-up procedures it would be fairly obvious if groups of pupils had received prior information.”

The National Primary Heads Association claims the pressure of league tables has led to teachers bending the rules by giving extra time to children or even indicating the right answers.