These latest problems follow the suspension of another major web-based development: School Profiles, which replace governors' annual reports.

Schools' access to those has been shut off while changes are made.

Data analysis

The old Panda reports were created by Ofsted to help its inspectors evaluate all the available statistics about schools' performance.

Realising how useful they were, it also made them available to schools - although head teachers differ in the extent to which they share the information with staff, governors and certainly parents, who may often not have heard of them.

The Department for Education and Skills (DfES) produces a "pupil achievement tracker", another tool for analysing the performance and progress of pupils.

It and Ofsted have been working on a system to combine these in a single interactive tool, which is Raiseonline. The Raise is an acronym for "reporting and analysis for improvement through school self-evaluation".

"It gives the flexibility to identify school specific issues and 'drill down' to investigate, at pupil level, the areas where improvement is most or least marked," Ofsted says.

Each school has access to data about its national curriculum tests and examination results, the progress made by its pupils and contextual information about the school.

They can update their own copy of the data to see how performance is affected, and can share this analysis as they choose.

'Ground-breaking'

Special reports draw attention to key messages arising from new "contextual value added" data about pupils' progress, which takes account of their backgrounds.

"There will also be an exceptions report that identifies the key issues for a school and takes the reader straight to the interactive environment to allow them to investigate these areas further," Ofsted said.

But - halfway through the autumn term - the project is behind schedule.

Ofsted blames this on its development being "a complicated, ground-breaking process".

Setting out its new target dates for data delivery, Ofsted said: "We recognise that the timetable below does not match our earlier expectations and that schools will not be able to use the system to any great extent this term for their self evaluation and target setting work."

It stressed that last year it was November before all schools had the updated performance data for the year. This time it is likely to be two months later.

Minimising disruption

"Ofsted and the DfES are ensuring that schools and other partners are aware of the revised timetables and can plan accordingly," a spokesperson said.

"In future years we anticipate that we shall be able to deliver the analysis to schools and inspectors earlier than in previous years."

Self-evaluation forms, now also done online, are fundamental to schools' ongoing improvement and preparedness for Ofsted inspections.

But head teachers have been told their access is currently "disabled" while improvements are made.

The spokesperson said a key consideration had been minimising additional workload and disruption for school managers, and the half-term break had been chosen as the best time to make the changes.

"We apologise for any inconvenience caused and have made best endeavours to minimise the impact of the transition."