Nearly a third of teenagers in Birmingham are on course to miss national standards for literacy and numeracy in their GCSE exams.
Results from key stage three tests taken by 14-year-olds show 30 per cent of them missed the expected level in both English and maths tests last year.
Hitting it is regarded as an indicator of whether they will go on to achieve the bench-mark target of between an A* and C grade at GCSE.
That, in turn, is viewed as key to future academic and employment prospects and earning power in their life beyond school.
Birmingham City Council defended its academic record, pointing out its improvement rate was above the national average in the key stage three tests.
Cabinet member for children, young people and families Councillor Les Lawrence (Con Northfield) added: "A number of schools in Birmingham provide for newly arrived children where English is not their first language.
"Schools are being very proactive and taking action to improve. However, often progress is made after Sats take place."
The impact of key stage three under-achievement was evident in last year's GCSE results for Birmingham - nearly half (47 per cent) of 16-year-olds failed to get a C or above in English; for maths it was exactly half and for science just over half (54 per cent).
Last night Liberal Democrat shadow Children's Secretary David Laws claimed the significant proportion of under-achieving 14-year-olds nationally was a Government failing.
"Ministers are now set to miss their unambitious target for all schools to have half of their pupils reaching the required level in English, maths and science," he said.
"In a majority of schools, more than a fifth of pupils are failing to reach the Government's benchmark in maths."
Mr Laws called on the Government to target under-performing disadvantaged young people.
Schools Minister Jim Knight yesterday announced a new resource pack would be made available for key stage three pupils to aid faster progress in English, maths and science.
He also outlined a raft of other measures designed to boost attainment at key stage three:
* new "innovative" materials for maths lessons for September;
* an increase in the teacher training bursary for maths teachers to £9,000;
* introduction of the first wave of the new vocational diplomas from September;
* free books for young children and investment in high-quality phonics materials.
Birmingham's grammar King Edward VI Camp Hill School for Girls was the best-performing in the region with all pupils achieving level six - one above the requirement - in English and maths and 99 per cent in science. It came sixth nationally for level six performance and second nationally for girls achieving level five. The most improved school for key stage three results regionally was Ash Green in Coventry while the best for value added - boosting the performance of pupils - was Birmingham's Waverley School.