The cross-party lobbying organisation said London boroughs need £740m to guarantee every five-year-old a school place over the next five years.
New research into the demand for school places, published by London Councils in its report Do the Maths, sets out the extent of the shortfall of capital funding.
It points to a growth in the capital's birth rate as the principal reason for a shortage of school places, noting that births have increased by 20.5 per cent since 2001/02.
Other factors include the economic downturn, a changing housing market which has resulted in a significant increase in the number of young children in many areas and a rise in the number of locally-born children requesting places at local schools.
Local authorities have said they are doing the best they can to provide places for all pupils by using temporary classrooms or expanding class sizes if they are allocated extra teaching resources.
London Councils' executive member for Children and Young People, Councillor James Kempton, said, 'We urgently need the Government to bring forward emergency funding to cover the current and expected demand for places, as well as change the way we are funded to prevent a situation like this occurring again.'
The report states, 'Without this funding, boroughs predict that the number of reception-age children in London without a school place could rise to over 18,300 before the middle of the next decade.'
- Further information: 'Do the Maths' is at www.londoncouncils.gov.uk/localgovernmentfinance/publications/dothemaths .htm