Free schools should be allowed to employ teachers without qualifications, London Mayor Boris Johnson has said.
Mr Johnson told BBC London: "Some of the greatest teachers in the world don't necessarily have the full set of qualifications."
His comments came as Deputy Prime Minister Nick Clegg said free schools should not have unqualified teachers.
Free schools, which can be set up by groups, are state-funded but operate outside local authority control.
"I think you need to be flexible, look at the individual and see what they have got to offer the pupils," said the mayor.
"I think back to my own childhood and the people who taught me and many of them didn't have an educational qualification at all, some of them were in Japanese prisoner of war camps... and they were fantastic."
This month, the head teacher of a primary free school in London, who was still studying for her postgraduate certificate in education, stood down following criticism from Labour councillors.
There was also controversy when the Al-Madinah free school in Derby was classed as inadequate by Ofsted inspectors.
The shadow education secretary has said that Labour will call for a vote in Parliament next week on teacher qualifications.
Tristram Hunt said: "The quality of teaching makes the biggest difference to learning.
"That's why Labour has been very clear: all teachers must be qualified."
The mayor's comments were made during the official opening of the Royal Greenwich University Technical College, in Woolwich, south-east London on Thursday.
Mr Johnson unveiled a plaque at the event alongside the Duke of York.