A survey of 21 schools in Croydon by the National Association of Head Teachers (NAHT) and the Secondary Heads Association (SHA) shows that they are employing staff who lack the necessary qualifications.
These include non-specialists, temporary, trainee teachers and those with a non-British qualification.
David Dibbs, head of Riddlesdown High School in Purley, said: "I have been a head for 18 years and I have never experienced anything like we have had this year.
"Two years ago we spent pound;3,500 on advertising. Last year it was pound;9,000. This year it has already been pound;15,000. We have to advertise over and over again for the same job.
"A few years ago in some subjects we would get 50 applicants. Now we might advertise four or five times and still get no applicant who is worth interviewing."
Riddlesdown has vacancies for four English and dramateachers, two science teachers, one music teacher and one RE teacher. There are nearly 900 teacher vacancies across London. Most of these are in maths, science, English, RE and modern languages.
In Croydon alone there are expected to be 85 staff vacancies in September. It was the first council to send headteachers abroad to fill vacancies and recruited more than 30 Australian teachers.
David Hart, NAHT general secretary, said: "The Government must have a much more cohesive recruitment strategy that will not just get good graduates but will retain teachers. It also needs to think seriously whether the existing recruitment incentives in London are adequate.
"I don't think we have started to tackle the root of the problem which is affordable housing in London."
John Dunford, SHA general secretary, said: "The new Government must give top priority to the teacher supply crisis which is facing so many schools, and to help schools use recruitment and retention allowances."