Estelle Morris's resignation is a real loss to education. She brought a refreshing honesty to the job. Her inner-city school reforms and teaching modernisation will be seen as her lasting legacy.

But once the Westminster lobby sees you as a liability, it is hard to recover. The A-level fiasco created that impression. Added to problems with criminal records checks (not of her making), it meant any later decision would be viewed in personal, not policy, terms.

This was true when she intervened in the Surrey exclusions row, though her decision may subsequently have been vindicated. And even if few A-level students were regraded, the factswere drowned out by a perception that many more were affected.

The irony is that there was probably no political pressure from Downing Street to go. Indeed many around the Prime Minister were keen to help her recover. She got a warm reception at the Labour conference. And most newspapers stopped short of calling for her to go.

However, Wednesday's discovery by the Tories of an unwise 1999 promise to quit if the literacy and numeracy targets were missed this year may have been the last straw. It overshadowed coverage of her genuinely radical solutions for reducing teacher bureaucracy and using support staff in schools. Yet most newspapers still reported her reforms, and they got a fair wind on TV and radio.

The crisis had taken its toll. She told of sleepless nights over A levels, and she wasn't exaggerating.

In the end, I suspect Estelle decided her reforms were more important than her. It was a typically self-effacing judgment from one of our least-spun ministers.

Conor Ryan was David Blunkett's adviser at the Department for Education and Employment, 1997-2001