David Bell said raising standards was his chief priority

David Bell said the new-style inspections, introduced this term, would help drive up standards.

There would be no quotas for the number of schools which would pass or fail, he said, but the public now had higher expectations of schools.

School inspections will be shorter and schools will receive less notice of inspections - typically just two days.

'Laying down a marker'

Speaking to the BBC News website, Mr Bell said schools should be asking serious questions about the performance of their pupils.

He said he could not rule out unannounced inspections, but that he would need to have "serious concerns" about a school before using this power for the first time.

Ofsted's previous seven-point scale of achievement will be replaced by four possible inspection outcomes - outstanding, good, satisfactory and inadequate - with a new set of criteria for each of these four grades.

Inspection changes Shorter notice - one or two days Shorter inspections - two days instead of five Schools' self-evaluation is central Simpler grading from 1 (outstanding) to 4 (inadequate) Children's views on school become more important

"We are saying to schools they shouldn't be surprised if things are tougher this year. We are laying down a marker," he said.

"We have seen schools in the past which have grossly over-estimated their performance, and that leads to some difficult conversations with the school.

"But inspectors always justify their judgements with evidence."

The Secondary Heads Association and the National Union of Headteachers said they welcomed shorter notice periods for inspections, which would remove the long period of anxiety for school staff.

Schools were previously given between six and 10 weeks' notice, and were inspected about every six years.

Inspections will now be carried out once every three years.

Inspections may be shorter, but they will be more sharply focused and will really get at the issues David Bell, chief inspector of schools

Mr Bell said he hoped the new inspection framework would reduce pressure on teachers, but that the main objective was always to raise standards.

"Schools should ask themselves whether their pupils are doing as well as they could.

"We'll be casting a laser light eye over their performance.

"Inspections may be shorter, but they will be more sharply focused and will really get at the issues."

He added that he felt it was time to learn from 12 years of Ofsted inspections and to refocus on what was important.

'Message of trust'

Self-evaluation by schools will be a major component of the new-style inspections.

Schools will produce their own report, which Mr Bell said needed to be "sharp, concise, and evaluative".

"It is important to send out a message of trust to schools," he said.

David Bell will be bringing forward his annual report to October

Inspectors will observe some lessons, with the aim of verifying the school's own evaluation, but will inspect fewer lessons than in the past.

"But inspectors retain the discretion to inspect whichever aspects of a school they wish," Mr Bell said.

"However, they will not purely focus on areas acknowledged previously as weak."

The pace of inspections will be intense at the start of the new school year, with 75 schools being inspected in the first week of term alone, and 900 by October half term.

Some head teachers say they would like inspectors to take greater account of the circumstances in which each school operates.

NAHT union leader Mick Brookes said a "more intelligent" system for taking a school's context into account was needed, and that similar schools should be compared.

Such factors as parental background and the level of higher education qualifications in the local community should be reflected, he said.

But Mr Bell said the new inspection framework had been "tried and tested" over a year-long pilot period and was welcomed by the majority of head teachers.

Inspections would focus on the value added by each and every school, he said.