A-levels became a grade easier in the Blair era

Sixth-formers gaining C grades in 1997 would get Bs if they took the exams now, according to a report by Civitas.

The think-tank says Mr Blair's schools legacy is 'best forgotten' because rising results in exams have been achieved using 'smoke and mirrors'.

Improvements have been largely manufactured by dumbing down standards, narrowing the school curriculum and forcing teachers to 'teach to the test' instead of achieving genuine gains in knowledge.

The report draws on Durham University research to claim a pre-Blair A-level is worth a grade more than a post-Blair A-level.

Taking an average of 40 A-level subjects, the study found that middle-ability sixth-formers would have scraped a C in 1997 but by 2005 were scraping B grades.

The academics from Durham's Curriculum, Evaluation and Management Centre concluded the exams have become 'more leniently graded'.

They also found that GCSE pupils are performing half a grade better than children of comparable ability in 1997.

The Civitas report warns that state schools have been brow-beaten into meeting targets at the expense of pupils' educational interests.

Schools have been putting pupils in for IT exams, which teach little more than how to send emails, because they are worth four good GCSEs in league tables.

A business studies GCSE paper in 2005 required only 47 per cent to gain an A*.

'While Blair inherited a poor education system, he has resorted to slapping a thick layer of paint on the rotten edifice and hoping that good presentation will do the rest,' said the report by Civitas's Anastasia de Waal and Nicholas Cowen.

'This particular "legacy" would be best forgotten.'

The report is the third in recent days to cast doubt on Mr Blair's 'education, education, education' pledge.

Analysis for the Sutton Trust charity found international tests failed to back up the Government's claims of rising standards.

And the Economic Research Council said that more grammar schools were needed to tackle 'dire' education standards.

The Civitas report says that the splitting in 2000 of the A-level into bite-sized chunks that can each be retaken had led to a sudden surge in grades.

The Durham study compared pupils' performance in an independent reasoning test with their grades at A-level.

A sixth-former scoring 50 per cent in the test in 1997 would tend to score a low C, but would get a low B in 2005.

Civitas claimed the 2000 A-level reforms meant they may not 'gauge the abilities of students accurately'.

It added: 'The consequence is that A-level exams have become so oblique at measuring academic ability that universities have been forced to come up with ways round this, from U.S.-style entrance exams to entire new qualifications.'