Inspectors will be given an extra year to inspect all schools under Ofsted's new light-touch framework as concerns are growing that many schools are poorly prepared for the changes.
Regulations show Ofsted will have to complete the next round of inspections by August 2009, despite its original promise that schools would be inspected on a three-year cycle.
A spokeswoman for the schools watchdog said the inspectorate would aim to inspect all schools by the end of the 2009 spring term.
The extended deadline is necessary to allow time to deal with any problems with the new system and to allow Ofsted to continue to conduct investigations into specific initiatives and curriculum areas, she said.
The new light-touch framework will be introduced in September and will place greater emphasis on schools to make their own self-evaluation.
Schools will be given only a few days' notice of inspectors' visits and Ofsted teams will be smaller and observe fewer lessons than they do at present.
Ofsted is understood to still be worried that many schools are poorly prepared for self-evaluation.
Pilot inspections conducted under the new arrangements have shown that some heads have used their self-evaluation to make assertions about the school's performance that are not supported by evidence.
Many schools have yet to complete their on-line self-evaluation forms, as is required by the new system.
In a letter sent to all schools last month, David Bell, the chief inspector, warned: "The new arrangements are very dependent on inspectors having their self-evaluation form available prior to the inspection and I would urge you to fill the form in, particularly if your school has not been inspected within the last four years."
In an interview with The Times this week, Mr Bell re-opened the debate about the Government's rejection of Sir Mike Tomlinson's proposals for a single diploma to replace A-levels and GCSEs.
Ruth Kelly, the Education Secretary, has insisted that "gold-standard" A-levels will stay with a new vocational diploma running alongside the existing examinations.
But Mr Bell said A-levels and GCSEs would be swept away by pressure from schools and colleges for change.
He said: "I just wonder if this is a good example of where practice might outstrip policy.
"If you go to schools and colleges they are beginning to do more of the things that were envisaged.
"The Secretary of State has quite categorically said GCSE and A-level are there, but three years down the line how does the qualifications structure fit with the practice?"
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