Inspectors now assess schools in a "short, sharp" fashion
More schools are being judged inadequate under a new inspection regime, figures from England's schools watchdog Ofsted show.
Half of the 2,140 schools inspected in the autumn were found to be either satisfactory or inadequate.
The proportion of schools classed as inadequate has more than doubled to 10%, compared with 4% in the 2008/2009 inspection period.
Ofsted said the findings reflected its "sharper focus on weaker schools".
Schools now have to achieve higher results to be good or outstanding under the new-style inspections, the first of which were carried out between September and December last year.
Children's Secretary Ed Balls said the government made "no apology" for Ofsted "raising the bar," but said a single term's figures were not a reliable indicator of performance for the year.
"It is absolutely no surprise that there is a higher rate of inadequate schools at the start of the inspection cycle - exactly the same happened when a tougher inspection regime was introduced in 2005," he said.
"Weaker schools are being specifically inspected more regularly and earlier in the inspection cycle to turn them around - while outstanding and good schools are now inspected every five years and satisfactory schools every three years."
The watchdog says its new-style inspections, the first of which were carried out between September and December last year, are part of a drive to raise expectations.
Only 9% of schools have been given the top rating of outstanding - compared with 19% of those inspected in the academic year of 2008/9.
Autumn inspections 2009
Outstanding - 9%
Good - 40%
Satisfactory - 40%
Inadequate - 10%
A total of 40% of the schools inspected were marked down as satisfactory.
Ofsted chief inspector, Christine Gilbert, said it was not appropriate to make direct comparisons over time. It was like comparing "apples with pears" she said, because weaker schools were being targeted under the new system and a greater number of those had been inspected last term.
"We wanted to concentrate more resources on the less effective schools, particularly those with pupils in danger of underachieving and offer clear recommendations for improvement," she said.
But John Dunford, general secretary of the Association of School and College Leaders, told BBC News: "I don't think it [Ofsted] is serving parents very well in the comparative views it is providing."
Ofsted also says it is typical in a new system that there are more schools getting poor results at first. The last time the inspection regime changed in 2005/06 the proportion of schools judged to be inadequate was also 10%.
Ms Gilbert said: "Every time an inspection framework is revised, expectations are raised and it is right for Ofsted to hold higher expectations on behalf of pupils and parents."
Of the schools inspected, 35% previously graded satisfactory were marked up to be judged good or outstanding.
Half of the schools inspected maintained the same grade as their previous inspection.
Ofsted said the reaction from schools had been "overwhelmingly positive", with nine out of 10 who had responded to feedback surveys saying they were satisfied with the way inspection was carried out.
Chris Richardson, head teacher at the Kings of Wessex School in Somerset, said the new inspection framework brought "significant improvements", particularly a sharper focus on student achievement.
"A new self-evaluation form meant we felt well prepared for inspection and it gave the inspectors a good overview of the school.
"Our students valued the opportunities to engage with the inspectors and give their perspective on the school."
But John Fairhurst, head of Shenfield High School in Essex, said the new inspection framework had downgraded his school from good with outstanding features to satisfactory, simply because the school's GCSE results in 2008 were weak.
"It's become a rather skimpy inspection with an altered agenda - heavily data driven," he said.
"The framework proved, in our case, superficial and underestimated the good work of the school."
Under the new arrangements, outstanding and good schools are only inspected once within a five-year period - previously it was every three years.
Satisfactory schools are inspected every three years and inadequate schools are visited regularly until they make the improvements necessary.
The new regime sees inspectors assess twice as many lessons and more emphasis is put on questionnaires filled out by parents and pupils.
Inspectors put more emphasis on pupil attainment - a point which has caused controversy, particularly for schools in challenging areas - rather than on improvement.
Schools are given two days' warning of an inspection and the inspections lasts for two days.
Liberal Democrat Shadow Schools Secretary, David Laws said: "Labour has had 13 years to get a grip on education, but thousands of children still attend schools which are not considered to be providing good standards.
"In spite of the controversy about whether these figures can be compared with earlier years, the bottom line is that half of schools inspected were not good enough."
Michael Gove, Shadow Secretary of State for Children, Schools and Families, said: "Today's figures show that thousands of children are being taught in schools that are not good enough".