In a U-turn announced just four days after a TES article raised concerns about schools conning Ofsted, the watchdog has said it will use unannounced "dawn raids" for all school inspections.
Sir Michael Wilshaw, Ofsted's new chief inspector, said that fear of underhand tactics highlighted in TES's cover feature, "Tricks of the trade" (6 January), is one of the reasons for the change, which is planned for September.
"If there is a level of concern that somehow a small number of schools are cheating the system, then Ofsted needs to address it and that is what we are doing," Sir Michael told TES.
Heads, however, have "real doubts" about unannounced visits leading to better inspections.
Brian Lightman, general secretary of the Association of School and College Leaders, said the move had come "out of the blue" without any consultation. "What worries me is that policy is being made on the hoof," said Mr Lightman, who is alarmed at the idea of the switch to no-notice inspections for all schools being triggered by the response to last week's article.
"That would be a very worrying overreaction," he said.
The article looked at concerns that schools were using underhand tactics, such as ensuring that difficult pupils and weak teachers were not around when inspectors visited.
A series of stories reported on the TES online forums included unruly pupils being paid to truant and schools passing around pre-prepared, laminated artwork when inspections were imminent.
The feature, which revealed that Ofsted had received 38 complaints about schools' conduct during inspections in eight months, prompted a huge reaction in the national media and led to more teachers revealing their own horror stories.
"It is disturbing to see Ofsted change its position in a matter of days and it suggests the policy has been created with an eye more to the sound bite than the evidence," said Russell Hobby, general secretary of heads' union the NAHT. "If a school could conceal evidence of widespread failure in just two days, then the whole concept of inspection is flawed."
Ofsted rejected a plan to make all normal school inspections unannounced in 2009, saying parents had complained that they would not be able to make their views known before inspectors arrived at a school.
The watchdog's new Parent View website means that parents can now give their opinions on schools all year round. But heads argue the site will be "unreliable" and "not necessarily representative".
This week, commenting on its consultation three years ago, Ofsted said it had found a "great deal of support for the move" to no-notice inspections. In fact, the consultation showed that more than three-quarters of heads and more than seven out of 10 teachers were against the plan.
No-notice inspections have already been introduced to schools causing concern, with some 1,500 "dawn raids" carried out in the past 18 months.
Sir Michael said that extending them to all school inspections was a "logical progression".
But Mr Lightman is annoyed that the change has been announced just after this month's introduction of the latest inspection framework. "Now inspectors are facing more changes and another consultation in which the outcome has already been decided," he said.
Ofsted said the move will form part of a wider consultation, but Sir Michael said: "I, as chief inspector, have got my mind made up on this one . Inspectors should see a normal school day, a routine school day, and any school that is doing its job properly will have nothing to worry about."
NOTICE PERIODS FOR SCHOOL INSPECTIONS
More than 52 weeks - 1992-2000
6-8 weeks - 2000-05
2-5 days - 2005-09
0-2 days - 2009-12
0 days - September 2012.
Original headline: The plot thickens as Ofsted plans `dawn raids' for all