The poll of 827 state secondary schools in England and Wales also found that heads thought staff shortages left them no choice but to hire those they regard as unsuitable.

The research - by the Times Educational Supplement (TES) and the Secondary Heads Association (SHA) - showed teacher vacancies were 25% up on the same period last year.

We are appointing staff who, in a perfect world, we would not touch with a barge pole

One head teacher surveyed

The heads surveyed were unhappy with 1,372 of the 7,127 appointments they had made.

If this is typical of all schools, then 6,000 of the 30,000 appointments in England and Wales are not up to scratch, the study concludes.

"We are appointing staff who, in a perfect world, we would not touch with a barge pole," the head of one north-west comprehensive told researchers.

A head of a comprehensive in the south-east appointed two people who "walked in off the street" with no qualifications.

Overseas staff

Other serious cases included a West Midlands comprehensive considering hiring linguists with no teaching experience and a Norfolk head unable to find a single supply teacher since February.

Many schools are having to appoint people who are unqualified or teaching a subject not their own, as well as overseas staff unfamiliar with the UK's education system, the TES concluded.

Mike Tomlinson says there are serious shortages

Such a situation will stifle the government's drive to raise standards in schools, it says.

The survey results come in the same week as chief inspector of schools in England Mike Tomlinson said teacher shortages were at their highest level for a decade.

Government ministers do not use the word "crisis" and say there are 12,000 more teachers than in 1998.

The Education Secretary, Estelle Morris, said Ofsted identified teachers as better than before, but she added some were having to teach unfamiliar subjects this term.

Untrained teachers

But TES editor Bob Doe said this was the worst period for teacher recruitment for 30 years.

"Head teachers are being forced to use untrained teachers or to offer jobs to overseas candidates without proper interviews or checks."

It's as much a retention as a recruitment problem

John Dunford SHA

SHA general secretary John Dunford said he was unsurprised by the survey results.

"About 5,000 vacancies and numerous unsatisfactory appointments will make the coming term very difficult for many schools."

Difficulties recruiting

Patrice Canavan, head teacher at Sion Manning School, told BBC News she knew of hired teachers who, "in a different market, would probably not even have been shortlisted".

Rosemary Campbell, head teacher of Northicote School in Wolverhampton, said it had become increasingly difficult to recruit and retain staff.

She has spent most of the summer holiday finding staff to fill vacancies for September.

"Sometimes what that has meant is going for graduates who are not teacher-trained and actually taking them on in the school and doing the training in-house."

The Department for education says efforts to tackle the teacher shortage are paying off.

A spokesman said: "Our focus remains on attracting and retaining more, top quality teachers to the profession.

"We have 12,000 more teachers in our classrooms today than in 1998 - more than in any year since 1984.

"That's not been a cause for complacency but a spur to continue with our strategy of recruitment, retention and reward.