MORE than 6 per cent of teacher posts were vacant in some education authorities at the start of this academic year, confidential figures sent to The TES reveal.
The figures are way above official statistics, which do not take account of the number of posts being covered by supply teachers.
Ministers claimed last week that the latest data, for January, will show a fall in the proportion of primary posts that are vacant in England, from 0.8 to 0.5 per cent and a slight rise in the secondary vacancy rate, to 0.8 per cent.
But the survey by Hounslow, Hillingdon, Brent, Slough and Hertfordshire reveals that they were coping with 568 empty posts between them. Hounslow's primaries had a vacancy rate of 6.8 per cent, with the authority as a whole short of 101 teachers (5.5 per cent).
Neighbouring Hillingdon was even worse off, short of 135 permanent teachers, adding up to 6.3 per cent of all posts.
Even the comparatively affluent county of Hertfordshire had a vacancy rate of nearly 2 per cent, equivalent to 172 teachers.
Philip O'Hear, Hillingdon's director of education, said a dozen of the authority's schools were "teetering onthe edge" of four-day weeks - a situation made worse by local agencies running out of supply teachers.
The authority is also facing a financial squeeze, with a cut in central government grants and a fast-expanding pupil population.
Moreover, its teachers are being drawn to jobs in neighbouring authorities who have Excellence in Cities programmes and education action zones that can offer extra pay in the form of retention and recruitment bonuses.
Mr O'Hear said: "All these factors make the situation in Hillingdon worse, and that's going to lead to a further failure to recruit and retain staff. We are rapidly going from a serious problem to something close to a crisis."
He said senior staff were spending valuable time trying to find bodies to put in front of classes, and being forced to put the needs of GCSE groups over younger pupils.
Problems of morale, poor pay, and long hours are among the factors that have contributed to recruitment difficulties, agreed three of the five surveyed authorities.
High housing costs in London and the South-east were exacerbating the problem (see story, right).
A spokeswoman for Hounslow said the borough was targeting "returners" to the profession, as well as trying to bring more people from ethnic minorities into teaching.