Police want to use search arches - similar to metal detectors used at airports - to catch people carrying knives into Oxfordshire secondary schools.

They have said the arches, introduced into the force last November, will also be set up at entrances to shopping centres and Oxford's bus and railway stations.

But details of when and where they will be used will not be publicised, so the measures retain a surprise element.

People would not be legally required go through these arches. But if police believe someone is carrying a weapon, they have the power to search them.

Officers have been in discussions with a small number of schools that have expressed an interest in the arches being set up at school gates.

They want to highlight the number of knives carried by pupils, as well as educate them about the dangers of carrying knives or bladed weapons in public.

David Wilson, headteacher of Faringdon Community College, said: "I would be in favour of police bringing this device into the school as a one-off, but I am not sure I would welcome this as a regular occurrence.

"We might get a one-off incident where a pupil may have forgotten to take a pen-knife out of a pocket, and brought it to school - but that might happen once a year."

Steve Lunt, headteacher of Oxford Community School in Glanville Road, East Oxford, said headteachers from across the city would meet police next week for a demonstration of how the scanning system would work.

He said many headteachers agree with the idea in principle, but added: "Most schools have three or four entrances and exits, so it would be very difficult to practically install. We have 1,100 students coming into school at the same time.

"I think this level of security is way off. Although we do occasionally have a problem (with students carrying knives), it is fairly rare. But we are, of course, happy to work closely with police on this."

Chief Supt Simon Chesterman, head of local policing for Thames Valley, said: "People seem to think carrying a knife keeps them safe, when in fact it could get used against them.

"If you carry a knife you are also running the risk of getting caught and being put before a court. You could get up to four years in prison."

Oxfordshire County Council said it fully supported the initiative.

Barry Armstrong, service manager for pupil attendance and welfare, said the authority hoped that the measures would act as a deterrent to people tempted to carry a weapon.

During the first trial of the search arches in Banbury, more than 1,600 people were screened at the Sound Exchange nightclub at the weekend.

Four small knives, one which an electrician had left in his pocket by mistake, and three pocket knives were confiscated.