Private security guards have been drafted in to patrol a school after a knifeman threatened members of staff.

Minders from MAN Security Services have been stationed at the gates to protect the 1,200 students at Varndean School in Balfour Road, Brighton.

Headteacher Andy Schofield said the protection firm was brought in after police failed to respond to a 999 call when the intruder entered the school grounds armed with a knife.

The same man had slashed a pupil in an assault out of school two weeks earlier.

Mr Schofield has written to parents saying the police had demonstrated their inability to provide an acceptable level of cover.

Senior officers had pleaded lack of resources and the school was not prepared to take any chances in the future.

Mr Schofield said: "Despite a 999 call explaining clearly the potential seriousness of events, they failed to attend.

"The incident could have resulted in serious injury or worse.

"Apologies and some promises of a better level of service have been made but we will wait to see whether the reality matches the promises."

Nigel Pinchin, operations director of MAN, said two officers from his Shoreham-based company would guard the gates throughout the school day.

He said it was the first time his firm, which also runs the Shopwatch scheme in Portslade, had been asked to provide cover at a school.

Mr Pinchin, whose daughter is a former Varndean pupil, said: "Primarily, our role is as a deterrent to stop unwelcome visitors from coming on to the site.

"This is the only school for which we provide cover but in the past 18 months we have been taking on more unusual jobs from factories to law courts.

"We're a very clean-cut company and proud of our reputation."

The school's move has prompted concern within the National Association of Headteachers (NAHT).

Eric Spear, NAHT president, said: "It sets a precedent once one school feels compelled to do it.

"We don't want armed guards on the gates like they have in America."

A Sussex Police spokeswoman said today: "There was an incident at the school which happened at 1pm. We received our first call at 1.40pm and were on scene by 1.53pm.

"A youth has been arrested in connection with the investigation into allegations of ABH.

"On the day of the later incident we received a call from the school but they didn't know at that time whether they wanted the police there or not. We were waiting for them to confirm exactly what was happening which is why we didn't go immediately."

She said the force had arranged a meeting with David Hawker, the city council's director of education, to discuss the policing of schools including truanting and drugs issues.