The plan to pilot no-notice Ofsted inspections has been dismissed as an election promise "drawn up on the back of an envelope" by a heads' leader.
Paul Whiteman, the general secretary of the NAHT school leaders' union, has warned that Ofsted arriving at schools unannounced will not help them to improve or be useful for inspectors.
He told a conference of school leaders in Manchester that the promise was developed by the Conservatives as a direct response to Labour and Liberal Democrats' plan to abolish Ofsted during the general election.
"I think the no-notice inspection announcement was policymaking on the back of an envelope," said Mr Whiteman. "I think the prime minister made that up so when the Labour Party and the Liberal Democrats went low saying they are going to abolish Ofsted, the Conservatives went high and said, 'Well, we will look at no-notice inspections'.
"It won't work. You know that and I know that. If we are going to look at school improvement, what we need to do is rebalance the relationship between inspection, compliance and improvement.
"The government has committed itself to a pilot but there was a pilot a couple of years ago and that showed itself not to work and a new pilot will show the same.
"You know that if an inspector walks through the gate of your school today, they are going to be sitting around having a cup of coffee while you get together everything they need to see."
But he said that feedback from schools about the initial phone calls that lead inspectors make the day before an inspection under Ofsted's new framework has been positive.
Mr Whiteman added: "It is working to set up inspection the next day. That is a pleasant surprise. But remember the telephone calls came because we petitioned Ofsted not to turn up on the afternoon as they had planned to do.
"I have already said to Ofsted that no-notice inspections won't work."
The union leader also questioned the Conservative plan to extend the inspection of secondary schools and large primaries to three days.
"Unless they resource Ofsted properly, the inspection teams are going to be under huge pressure and then we would have worry about the consistency across the inspection process," he said.
During the general election campaign, the Conservatives announced the plan to trial no-notice Ofsted inspections and extend the length of inspections at secondary schools and large primaries to three days.
The party pledged an extra £10 million as part of the announcement, which came after its manifesto was published.
At the time, the Conservatives contrasted their plans to strengthen Ofsted with Labour’s plan to scrap the inspectorate.
Since the election, education secretary Gavin Williamson has spoken about the importance of strengthening Ofsted, announcing plans to remove the exemption on "outstanding" schools being routinely inspected.
He said: “Parents want to know that they are making the best, most informed choices about their children’s education. Making sure that all schools are regularly inspected means they will benefit from the expert insight Ofsted provides when making these decisions.
“We know parents trust Ofsted – and with good reason. It serves a valuable purpose as the only organisation that gives a clear, accessible and impartial view on school and college performance."
The Conservatives, Ofsted and the Department for Education were contacted for comment.