The Office for Standards in Education (Ofsted) believes that while exam results at a number of schools may appear good, they ought to be much better - and many pupils, particularly the brightest, are losing out.

Under a new inspection framework which will come into force in January, these schools will officially be designated "under-achieving".

The category is intended to issue a warning about a school's future to teachers and parents. Failure to improve after as little as a year could designate a school as one with serious weaknesses, or even one that is failing.

Revealing details of the new framework on Tuesday, senior Ofsted officials acknowledged that some "coasting" schools could have previously been missed.

Mike Tomlinson, Ofsted's director of inspection, said: "What we are trying to do is to make their identification more systematic so they can be forced to take action to improve."

Chris Woodhead: "We can reduce the burden on successful schools"

Currently, between 3% and 4% of schools are branded as failing, and between 8% and 10% are deemed to have serious weaknesses.

These figures are close to Ofsted's initial estimates of how many schools would fall into these categories.

But the watchdog has not revealed how many schools it expected to designate as "under-achieving".

Chris Woodhead, Chief Inspector of Schools in England, said: "Now that we have inspected all schools, we can reduce the burden on successful schools, and focus instead on driving up standards in schools that are failing, have serious weaknesses or are just coasting along."

Under the new framework, about one in five schools will undergo "light touch" inspections.

Schools which appear to be doing well will still be inspected once every six years, but will be visited by fewer inspectors, who will spend less time carrying out their inspections.

But Ofsted has insisted that a school could still "fail" a"light touch" inspection, and where schools were found to be doing less well than had been thought, a full inspection could follow within a year.

Pupil mobility

Other new measures include giving schools a shorter notice period before inspections take place. From January, all schools will only receive from six to 10 working weeks' notice of an inspection, instead of two terms.

Inspectors will look for evidence of racial harassment in schools for the first time.

They will also collect evidence about pupil mobility in schools. This is an issue often raised by teachers in inner city schools, who say they cannot be judged against national standards when children may only spend weeks with them.

Mr Tomlinson denied that in acknowledging the impact of pupil mobility, Ofsted was accepting excuses for failure.

"There are schools that vigorously seek to overcome these difficulties. If a school just sits back and says ... there's nothing they can do about it, that is just accepting an excuse," he said.

Nigel de Gruchy, general secretary of the National Association of Schoolmasters Union of Women Teachers, attacked Ofsted's new focus on "coasting" schools.

"There seems to be no end to the number of schools Ofsted is prepared to menace and threaten," he said.