Headteachers are only just managing to avoid the worst scenario of sending pupils home because there is no teacher to take lessons, by increasingly relying on under-qualified, short-term and supply staff.

Other strategies include pushing up class sizes, asking teachers to teach outside their specialist subject areas, reducing non-contact time and even dropping subjects such as technology altogether.

One head reported, on hearing that a teacher had just narrowly missed out on a job at a neighbouring school, of asking her to be bundled into a taxi so that he could interview and recruit her straight away.

Professor Alan Smithers and Pamela Robinson, of Liverpool University, who carried out interviews with 107 heads and surveyed 923 schools for the National Union of Teachers, also found that the desperate trawl for staff ws coming at a cost to heads and teachers.

Heads were finding the search stressful and time-consuming, while demands on teachers increased if a vacancy remained unfilled, causing staff to shout at children and leading to tiredness, illness and absence.

Forty-nine per cent of the 923 schools surveyed reported finding it difficult to fill vacancies for full-time staff in 1999-2000.

The problems were not confined to London. Although primary schools in the capital reported the most problems finding staff, in the secondary sector there was little variation across the country.

There was anecdotal evidence that schools in special measures found recruitment harder.

Doug McAvoy, general secretary of the NUT, said the report highlighted the Government’s attempts to boost recruitment to teacher-training and make the profession more attractive as a “quick fix”.

He added: “The answer is decent, competitive salaries, smaller class sizes and more support time during the school day.