The former national schools commissioner has told Ofsted to stick to being a regulator and criticised its plan to get more involved in school improvement.

Sir David Carter is concerned about the inspectorate’s plan to trial a new, longer inspection aimed at helping underperforming schools to improve.

Ofsted has said it wants to carry out a new type of inspection “to increase the depth of diagnosis” it gives to what it calls “stuck schools” that have been rated as less than good for more than 10 years.

Ofsted: Plans for non-judgemental visits to ‘stuck’ schools

Challenges: Five problems facing so-called stuck schools

Exclusive: Sir David Carter on why he is standing down

The regulator has said it is well placed to help these schools to diagnose their problems and is now calling for more funding from the Department for Education to trial the longer, deeper inspections.

It has said the inspections would have “the aim not of passing judgement but of enabling support to improve”. However, Sir David, who stepped down as national schools commissioner in 2018, questioned the plan.

Posting on Twitter, he said: “Just be the regulator. Leave the improvement to others. That way there is no confusion. [The] last thing we need is another pilot to add more pressure to leaders and governors getting on with school improvement.”

In another post, he added: “What these schools need is a break from frequent inspections to give their trusts time to improve them.

“Ofsted should not be doing school improvement - they should be forming views about how effective the improvement plan is. How much new Ofsted thinking can the system cope with?”

Chris Jones, Ofsted’s corporate director of strategy, also commented on the plans on Twitter.

He said the inspectorate was not planning to provide the school-improvement support.

Mr Jones added: “We want to enable the support to succeed by helping the school with a more detailed understanding of its particular strengths and weaknesses.”

Responding to one of Sir David’s tweets, he said Ofsted wanted to carry out pilots to see what worked.

Sir David responded to say the issue was that “you cannot have an organisation that one week is the regulator then the next wants to be part of the improvement”.

His intervention has echoes of a Whitehall turf war fought with Ofsted during the last year of Sir David’s spell as DfE national schools commissioner in 2018.

There was controversy then over whether the role of Sir David’s regional schools commissioners was overlapping with Ofsted - with the two bodies carrying out parallel systems of checks on schools.

In May of that year, the former education secretary Damian Hinds sided with Ofsted and announced that regional schools commissioners would no longer conduct shadow inspections of schools.

The commitment was part of a plan to lessen the burden of the accountability system.

The plan for Ofsted to carry out new “stuck school” inspections was revealed in a report published on Wednesday.

The watchdog has identified 415 “stuck schools”, which it says have been in a cycle of low performance despite receiving inspections and support designed to help them improve. These are schools that have not been rated good or better since September 2006.

When the report was published, Amanda Spielman, Ofsted’s chief inspector, said: “Our inspectors have found that the majority of schools in challenging areas are providing children with a good education that sets them up to succeed in later life.

“What the remaining stuck schools need is tailored, specific and pragmatic advice that suits their circumstances - not a carousel of consultants. They are asking Ofsted to do more to help, and we agree.”

An Ofsted spokesperson said: “Ofsted has recommended that the government funds us to trial a longer, deeper inspection approach with some of these stuck schools, with the aim of not passing judgement but of providing a more in-depth diagnosis to help those following up with the improvement work.

“We will pilot this approach to see if it works - it’s early days but, to be clear, we will not provide the support work.”