Test results at more than half the state primary schools in Brighton and Hove were below the national average.
The tests – known as SATs – are taken by children at the end of key stage 2 as they near the end of their final year at primary school.
Provisional figures suggest that 25 out of 48 primary schools underperformed in the reading, writing and maths tests – up from 18 schools last year.
Brighton and Hove City Council said: “We will be exploring the results of these schools further.
“Schools with very low progress have plans to improve that are monitored by a school improvement board.”
The proportion of children to reach the ‘benchmark standard’ was 66 per cent – one percentage point higher than the national average of 65 per cent.
The figures are set out in a report going before the council’s Children, Young People and Skills Committee next Monday (September 16).
The report on standards and achievement in Brighton and Hove schools and colleges said that phonics results for youngsters starting in Year 2 dipped from last year.
The report also said that ‘early years’ results had slipped year on year, with literacy listed said to be ‘the main area of challenge’ for children starting in Year 1 this month.
The provisional figures show a slight fall in the proportion of five-year-olds achieving a ‘good level of development’ in the Early Years Foundation Stage.
In Brighton and Hove 71.5 per cent of the 2,639 children in the reception year achieved this level at the end of the last school year, slightly below the national average of 71.8 per cent.
This result is down from last year when 72.6 per cent of the 2,656 children in Brighton and Hove achieved a good level of development.
Pupils in Brighton and Hove tended to start doing better than the national average at the end of key stage 1, when they were seven years old.
The report said that there were no national figures with which to compare GCSE performance although the proportion achieving a pass in both English and maths rose by one percentage point locally this year.
Other measures of GCSE results locally appear to have dipped slightly.
The picture is brighter at A level, with an above average number of passes and notably so at the highest grades.
About 65 per cent of pupils take their A levels at Brighton, Hove and Sussex VI Form College (BHASVIC), 24 per cent at Varndean College and most of the rest at Cardinal Newman Catholic School.