More than 20,000 teachers don't have the right qualifications to teach their subjects, official figures show.
The figures include those being fast tracked into the profession and foreign teachers whose qualifications are not recognised in the UK.
It stood at just 3,000 when Labour came to power in 1997.
In some areas more than 10 per cent of teachers do not have right qualifications and levels were worst in Inner London boroughs.
Hackney, with more than 15 per cent, had the highest proportion of all 150 education departments in the country.
A Hackney spokesman said: "We have a small proportion of highly qualified overseas trained teachers whose qualifications are not recognised in the UK.
"We support all our overseas trained teachers to gain qualified teacher status and most achieve this within a year.
"These teachers, like their peers, bring valued skills and experience."
The Department for Children, Schools and Families said the "unqualified" teachers have passed a Criminal Records Bureau check before practising.
Computing, religious education, business studies, classics, design technology and drama have the highest numbers of unqualified teachers.
Those with the least unqualified teachers are physics, biology and chemistry.
Overseas teachers can work unqualified in England for up to four years without the need to gain qualified teacher status.
Ministers said last week that thousands of 16-year-olds with basic qualifications should be given jobs in schools, helping to teach pupils just two years younger than them.
Ed Balls, the Schools Secretary, said as many as 4,000 positions could be created for them to work as apprentice teaching assistants, supervising pupils aged from five to 14.
The Conservatives have suggested more pay and perks for the best staff because teacher quality is the "biggest single determinant of a child's success". (By Nick Allen Last Updated: 2:28PM GMT 08 Feb 2009)