George Balmer handed in his notice when he learned he was being asked to take a class of more than 30 at Cuddington Croft primary school in Cheam, near Sutton.
Headteacher Mike Farnham admits the average class size this year will be 31 in order to find extra money for classroom assistants and buildings. He says the figure had been agreed with staff.
But the row takes on an ironic air as the Government prepares to fulfil its election pledge to cut numbers in infant classes to 30 or below. Labour's first Act since the election has already been signed, abolishing the Assisted Places Scheme to fund a cut in infant classes.
Mr Balmer's is a lone stand. The Year 2 teacher, who does not belong to a union, claims he was given an assurance over class sizes when appointed a year ago - which Mr Farnham denies.
"I taught 33 at my last school in Tooting, and I became very aware of the difference the extra numbers made - 30 really is a critical number," Mr Balmer said. "At 30, you can do your job; at 31 you can't. Somebody has to take a stand, otherwise it goes to 31, 32, 33. " Mr Farnham said: "The resources of a school, both human and physical, depend on numbers. The governors decided that to meet the school's aims and development plan for the forthcoming year that would be necessary.
"Teachers have been involved in discussions pretty frequently over whether they would prefer to operate at 30 or 31, and the overwhelming view was they would rather have 31 and the extra resources, particularly classroom assistants. "