A study by the Office for Standards in Education comparing the inspection reports and test results of 4,000 primary schools has found that the two measures of school performance can produce conflicting evidence.

As an example, the study found that higher levels of achievements were recorded in tests in English than in the inspection reports for the subject. There were also 30 schools which received commendations from inspectors and then scored below average marks in pupils' tests.

This lack of a correlation between inspections and tests has prompted the Liberal Democrats to question the value of the two systems of checking schools' effectiveness.

Ofsted explains differences

"The question that this raises is why are we spending all this money, when either the inspections or tests are not working. There is an inherent contradiction here," said Mr Foster.

The Liberal Democrats' spokesman also pointed to Ofsted's recognition that in future inspectors would be required to report on schools only "when they have sufficient evidence to come to a secure judgement". Don Foster said that this begs the questions as to whether judgements were reached in the past without "sufficient evidence".

Ofsted has suggested a number of reasons for differences between inspections and test results. The inspections agency said that pupils' attainment might have changed between the times of an inspection and testing; results have improved in recent years encouraging inspectors to have higher expectations and that inspections take a view of schools that is broader than the areas covered by tests.