Almost 10,000 Birmingham children are still being packed into primary school classes of more than 30, the Government has admitted.
Official figures from the Department for Education revealed 9,991 primary pupils in the city, one in 11 of the total number, are taught in classes of 31 or more.
And four primary classes in the city have more than 40 pupils.
Tony Blair made smaller class sizes a key issue when he swept to power in 1997, using the battle cry of " education, education, education".
He promised to abolish classes of more than 30 for children aged five to seven.
But although the official figures show this pledge has largely been kept, class sizes shoot up again after the age of seven.
The result is that across the West Midlands, one in seven primary pupils are taught in classes of more than 30. This includes 5,780 in Shropshire, or almost one in four of the county's primary pupils.
And it includes 9,327 in Warwickshire, more than one in five of the total.
In Solihull, 3,027 pupils, almost one in six, are taught in oversized classes.
And in Dudley, five primary school classes with just one teacher contain more than 40 pupils.
Research by the University of London in 2003 found that smaller class sizes can make a significant difference to a child's achievement at school.
The importance of smaller classes is greater at a younger age, and pupils who have been struggling are particularly helped.
A Department for Education spokesman said: "We have made great strides in reducing infant class sizes.
"In 1997 some 23.9 per cent of five to seven year olds classes had 31 or more pupils compared with 1.2 per cent in 2005."
But Edward Davey, Liberal Democrat education spokesman, said: "Large class sizes harm standards, effect discipline and are a major barrier to improvement in schools.
"All research in this area shows that for younger children, especially those who struggle in school, smaller classes are vital.
"Parents who can afford to opt for private education know that - because a main motivation for them has always been smaller class sizes."
A spokesman for Shropshire County Council said: "Increased funding would better support learning for all children, enabling headteachers to organise classes with qualified teachers and effective teaching assistants to best suit the learning needs of children to help them perform well."
A Birmingham City Council-spokesman said very few pupils under seven were in larger classes, but there was no official requirement to keep class sizes at 30 or less for older pupils.
He said: "At junior level there is no Government requirement not to succeed 30 pupils, and providing the accommodation and teaching provision is sufficient there are no DfES concerns."
Across the country, more than 15 per cent of primary school children in England are still taught in classes of 31 or more.