The Education Secretary Michael Gove is coming under pressure to raise class sizes for five-year-olds as a result of the public spending squeeze and a rise in the birth rate.
Liberal Democrat-controlled Sutton Council is canvassing support from councils to urge Mr Gove to scrap legislation limiting class sizes for five to seven-year-olds to 30.
It says it cannot afford to provide the extra classrooms it would need because of a rise in the birth rate.
In a letter to all councils in London, Niall Bolger, Sutton's chief executive, said: "All London boroughs are facing unprecedented demand for additional primary school places." Last September it had to spend £7m on extra classrooms to stay within the law. It is suggesting the maximum class size be raised to 32 – a move which would have avoided the need for the extra spending last year.
Mr Bolger said the rise was necessary "in order to enable councils to meet their statutory obligations to educate all their young citizens within their financial envelope".
The move has already been criticised by Merton Council education spokesman Peter Walker who said: "Increasing class sizes will threaten school standards, is unfair to our children and will endanger our economic prospects."
A spokesman for the Department for Education said: "The law remains clear that it is illegal for infant classes to exceed 30 pupils.No parent would want their child taught in a huge class."