HUNDREDS of Norfolk schools were urged last night to double-check their computer security after five high schools had their systems infected by a virus.

HUNDREDS of Norfolk schools were urged last night to double-check their computer security after five high schools had their systems infected by a virus.

Thousands of pupils were unable to access important online work after the invasive programme breached online security and quickly spread through the ranks of computers in hundreds of classrooms.

Teachers and software experts had to work around the clock to get rid of the Downadup/Conficker worm, which has the potential to get hold of sensitive information and circulate it.

The worm - which also hit Norfolk County Council's professional development centre and sensory support team - has infected millions of computers worldwide, including systems within the French air force and navy and the US air force.

The council was able to cut off the schools and the two other areas from the rest of the network before it spread further, but it is not known whether any top-secret information was compromised by the attacks.

Downadup is believed to be such a threat that software giant Microsoft has offered a $250,000 reward to anyone coming forward with information that leads to the capture of the person who wrote the programme.

The high schools affected include Caister and Oriel at Gorleston.