Ofsted has backtracked on its plans to carry out no-notice inspections after harsh criticism in a consultation that closed this week.
Proposals to pilot unannounced visits have been called into doubt after being rejected as impractical by inspectors, The TES can reveal.
Aspect, the union that represents the majority of inspectors, said arriving at the school gates without any notice could lead to problems securing time with senior teachers.
The idea was also dismissed by teachers' leaders, who said it was unworkable, would increase stress, and that it displayed a lack of trust in school staff.
The criticisms were made in response to a consultation on inspection regime changes, which are due to be introduced next September.
Aspect said that the current two days' notice allowed for "the reduction of stress and over-preparing".
"Unannounced visits could prove problematic in terms of accessing what inspectors need, especially meetings with senior management. On balance, therefore, this possibility is not supported," it said.
Christine Gilbert, chief inspector, first proposed no-notice inspections in December last year, saying that parents and pupils had called for "drop in" visits "in order that we see the school as it really is".
When the consultation launched in May, it was announced that trials of zero-notice inspections would follow in the next 12 months. But this week Ofsted refused to confirm that pilots would go ahead.
A spokeswoman said: "It is not possible at this stage to give a definitive answer as to whether we will be conducting no-notice inspections. The consultation only closed on Monday and we will be considering responses before coming to a conclusion."
John Dunford, general secretary of the Association of School and College Leaders, said: "Unannounced inspections assume that a school has something to hide. It shows a lack of trust in the professionalism of school leaders."
The National Association of Head Teachers said it was "unacceptable" that inspections could take place on a day when headteachers were not in school.
Zero-notice visits would increase pressure on schools, the union added, causing them to be constantly checking they were ready for inspectors, rather than concentrating on high-quality teaching.