The proposal was one of several new education policies launched this week by David Cameron, the party leader, in a report which also recommended national reading tests for six-year-olds.
The Conservatives' schools "green paper" expressed concern about Ofsted's data-driven, lighter-touch regime, introduced in 2005, which means not all teachers are observed by inspectors.
It promises Tory ministers would extend the inspectorate's powers to enable more detailed and longer inspections. It claims that all full-time teachers were inspected in the past, something Ofsted denies.
The report said: "We will also consult on introducing lightning inspections to minimise the time taken up by in-school preparation and to secure a more accurate snapshot of teaching, with classroom inspections taking place unannounced, without the prior knowledge of the teacher."
Steve Sinnott, NUT general secretary, said: "This is back to the future - the re-introduction of a heavy-duty, high-stakes operation that will do nothing for teacher motivation and the attractiveness of the profession."
The report was also criticised for proposing that Ofsted should ensure all schools were using synthetic phonics, and setting pupils in all academic subjects by ability, while advocating decentralisation.
John Dunford, Association of School and College Leaders general secretary, said: "It is hugely ironic and disappointing that a political party which says that it wants to free heads from government control is telling them how to organise their schools in this detail."
Mr Cameron countered accusations of a contradiction by saying immediate action was required. In the long run, he said he wanted to create a generation of schools answering to parents rather than ministers - seemingly implying that they would be free to select their own teaching methods.
He also emphasised his party's view that traditional discipline and dress was the key to success in comprehensives.
The Tory leader launched the report at Mossbourne Academy in east London, a school which was opened and praised by Tony Blair.
It was a clear attempt by Mr Cameron to seize the initiative on the academies programme - he has already accused Gordon Brown of rowing back on it.
This week, a report from the centre-right Policy Exchange think-tank claimed academy sponsors were becoming disenchanted with the programme because it had been weakened by tighter curriculum controls, standardised building projects and co-sponsorship with local councils.
The Conservatives' new proposals would answer many of those criticisms. They recommended changes in planning rules to make it easier to find land for building academies, a restoration of curriculum freedoms and preventing local councils from stopping them opening by saying they would create surplus places.
The party would also drop any requirement for organisations without capital reserves - including parents groups - to find sponsorship money to set up academies, which in future would be smaller.
The Conservatives said the construction of their academies would be funded by diverting pound;4.5 billion from the Building Schools for the Future programme. Labour said this would mean scrapping more than one in seven secondary school rebuilding projects.
Asked how any surplus places would be funded, Nick Gibb, shadow schools minister, said there might be some "overhead and unit cost issues" but the majority of education funding was allocated per pupil so it would make little difference.
But the NUT said it would mean a "massive" cut in school funding.
- 'Raising the Bar, Closing the Gap' is at www.conservatives.com
The Conservative education policies include:
- Replacement of key stage 1 English assessments with a national reading test for six-year-olds;
- All teachers to be inspected by Ofsted without prior warning;
- Ofsted to ensure all schools teach synthetic phonics effectively and set all academic subjects by ability;
- Discipline to be encouraged through "credit" and "debit" reward schemes with cups, trips or book tokens for good behaviour and the withdrawal of privileges for bad;
- 223,022 extra school places within nine years in new smaller academies, on top of the Government's existing commitment for 230 academies by 2010;
- Local authorities must look at dividing large failing schools into smaller schools on the same site.
- Extra funding attached to disadvantaged pupils to encourage schools to take them - but no details of how much or where it would come from.