Cameron works as a teaching assistant

The Tory leader travelled to Humberside to find out for himself the problems facing teachers in secondary schools by taking part in lessons.

He kept his visit secret but took with him a cameraman to record his experiences on his website, WebCameron.

A spokesman said his aim was to experience what it was like to teach a class full of teenagers, as well as hear what teachers say about testing, government bureaucracy and funding.

The Evening Standard recently revealed that Mr Cameron sees his target key voters as "the staff room of an ordinary primary school" - symbolising his commitment to public services.

He wants to convince voters that he is serious about tackling deep-rooted problems in public services and society, issues where the Conservatives were traditionally seen as weak.

Last week, he lived for two days with a Muslim family in Birmingham.