The Tories provoked a storm of criticism last night with their latest election broadcast which depicted children as drug-taking, vandalising thieves.

The Conservatives said the intention of the broadcast was to highlight the effect of teacher shortages.

The ad, which runs through a school day, begins with a shot of an empty classroom and the line: "Teacher shortages at their worst for ten years."

It goes on to show children shoplifting, setting fire to a car and spraying graffiti on a railway bridge.

Doug McAvoy, general secretary of the National Union of Teachers last night condemned the broadcast as "off the wall" and "a dreadful misrepresentation" that has "alienated at least two significant blocks of voters: parents and teachers".

Education secretary David Blunkett said the ad "shows how desperate the Tories have become".