His admission will reignite concern over staff shortages in the capital which have seen schools recruiting teachers from Africa, Australia and the far East.
Schools in the capital rely heavily on staff from overseas and on trainees.
Shortages of qualified staff in subjects such as maths and science have forced schools and local authorities to look abroad.
But while the number of qualified secondary teachers has increased slightly since 1997, the number of qualified primary teachers has fallen by 5 per cent.
Overall, there are more than 5,400 teachers without qualified status in London schools out of a total workforce of 52,500.
Mr Miliband's admission came in a Parliamentary answer to Simon Hughes, Lib Dem MP and London mayoral candidate.
Ministers have attempted to tackle recruitment problems in the capital by introducing higher pay for inner London teachers. The strategy for the capital, drawn up by Tim Brighouse, its schools "tsar", includes a new chartered London teacher certificate to encourage staff to stay.
But the National Union of Teachers accused the Government of exploiting falling rolls to cut qualified teacher numbers. "There is a poisonous combination of teacher shortages and government policy reducing the number of qualified teachers," said John Bangs, NUT head of education.
A spokeswoman for the Department for Education and Skills said: "There are more teachers in London than ever before and the vacancy rate has fallen from 1,356 in 2002 to 1,020 in 2003. She said measures to attract the best teachers to the city included subsidies to buy homes and increased pay.