Nearly half of England's local authorities could face sudden inspections from May under Ofsted's new drive to improve the support that councils offer schools, TES can reveal.
The majority of triggers for the visits, for which the watchdog will only give up to five days' notice, are based on national averages and therefore automatically expose a large proportion of the country's town halls to the extra scrutiny.
One of the measures - having a lower than average number of pupils attending good or outstanding primaries - will instantly place 72 authorities, 48 per cent of the total, into the inspection bracket.
Ofsted has said that the information it gains from the inspections could help education secretary Michael Gove determine where to use his intervention powers, which he can apply when he judges schools are not up to scratch.
The ATL teaching union has claimed that introducing local authority-wide inspections is a "highly political" plan designed to lead to the creation of more academies. Local authorities argue that increasing school autonomy and academy conversions are hindering their ability to improve schools.
But chief inspector Sir Michael Wilshaw said that the action is necessary. "If England has any pretentions to be a world-leading education system, we must have higher ambitions and be absolutely committed as a nation to doing something about the wide variations in standards across our country," he said.
Inspectors, who will visit authorities for about five days, will look at the effectiveness of their leadership on school improvement, the extent to which they know their schools and where they focus their support. They will also examine the effectiveness of that support, the rate of school improvement and the extent to which authorities broker assistance from other schools.
Ofsted's last annual report highlighted stark regional differences in school ratings between authorities. In Derby, for example, just 43 per cent of pupils were attending good or outstanding primaries, compared with 92 per cent of pupils in Camden, North London.
Sir Michael described such variations as "unacceptable", saying: "Why should some children get a much better deal than others in another local authority with similar demographics? It's an issue of equity and social justice."
Last month, Derby became the first in a series of authorities to experience Ofsted descending for a blitz of simultaneous inspections of its schools, partly to determine whether the authority should itself receive a full inspection.
TES understands that, of the 10 schools inspected in Derby, seven were found to be good or outstanding, with three requiring improvement.
Ofsted last inspected local council education services in 2004-05. The old regime, introduced when Labour was in power, led to the partial or full privatisation of several authorities, including Bradford and Islington in North London.
In other local authorities - including Hackney in East London, where Sir Michael worked as the headteacher of Mossbourne Community Academy - education services were outsourced to not-for-profit organisations.
So far, under the new system, Ofsted has only said that authorities deemed not to be providing an acceptable standard of service will be subject to reinspection.
The new inspections will not use the watchdog's usual four-grade rating system and will instead give a simple pass or fail verdict.
David Simmonds, chair of the Local Government Association's Children and Young People Board, said: "Local authorities want to be able to intervene more quickly in underperforming schools, but we are prevented from doing so as a result of decades of reforms to give schools greater independence and reduce what was perceived as council interference.
"Councils now lack the direct powers that academy sponsors and the government have to take quick and decisive action when schools are underperforming."
Mary Bousted, general secretary of the ATL, said: "Ofsted's plans are highly political, putting local government under pressure to aid the proliferation of academies. Since the responsibility for academies lies with the secretary of state and not local authorities, will Ofsted be announcing a framework to inspect Mr Gove?"
Inspections of local authority school improvement services may take place "where one or more of the following apply":
The proportion of children attending schools rated good or outstanding is below the national average.
A higher than average number of schools are in an Ofsted category of concern andor there are indicators that such schools are not making rapid improvement.
A higher than average proportion of schools have not been rated good or outstanding.
Attainment is below national levels.
Rates of progress are below national levels andor the trend of improvement is weak.
The volume of complaints to Ofsted about schools in the local authority area is a matter of concern.