As an urgent inquiry begins into allegations that exam boards fixed grades, BBC education correspondent Mike Baker examines the background to the most serious crisis faced by the Education Secretary Estelle Morris.

He says the problems arose in part out of recent changes in the exam system.

The key to understanding this row is the attempt to ensure the new, two-part A-level - taken for the first time this summer - maintained the same standard as the old one.

But it's like comparing apples and pears.

The exam authorities knew the new system was likely to lead to much higher pass-rates at full A-level.

Estelle Morris: Under pressure

That's because the new AS-levels, taken after one year, effectively filtered out the less able students at the half-way stage.

Not surprisingly, those with poor AS grades were reluctant to attempt the full A-level.

Yet at no stage did ministers or exam chiefs try to prepare the public for this outcome. Indeed they probably feared a sceptical response to pass-rates approaching 100 percent.

The independent inquiry will be looking into whether this concern led someone, at the last minute, to adjust the A2 grades to avoid embarrassingly high pass rates.

If the exam boards adjusted coursework grades for these reasons - as opposed to simply trying to keep parity with the old A-level - that would clearly be both misleading and unfair.