Across the capital, only 16 per cent of primary school classes have more than 30 pupils - roughly half the proportion in the rest of the country. One class in five in London has fewer than 20 pupils.

But those figures mask a huge variation between authorities. Just 1.5 per cent of classes in Kensington and Chelsea are over 30 and no inner-London authority tops 9 per cent of its classes. But in Kingston-upon-Thames, it is 54.1 per cent.

In metropolitan boroughs outside London, 29 per cent of primary classes are over 30, compared to 28 per cent in shire counties and 27 per cent in the new unitary authorities. The national average, including London is 25 per cent.

The Audit Commission figures relate to all primary school classes, including key stage 2. Labour’s pledge that no child shall be in a class over 30 by the end of its first term relates only to key stage 1. Future reports are expected to include a figure for infant classes only.

The Government can expect next year’s figures to be keenly scrutinised as they will cover Labour’s first year after being elected on its class sizes pledge. But it will be 2000 before figures are published which reflect the pound;22 million the Government is investing in tackling class sizes in the coming year.

Kingston-upon-Thames is also typical of authorities with high key stage 2 test results but also high class sizes. That suggests that despite vocal parental demands for smaller classes, they are prepared to accept large classes in schools with good results.

Nicolas Barnard