Ofsted has warned of a "shocking" fall in disadvantaged pupils’ GCSE results in Reading.
The borough has had a 6.9 percentage point drop in the proportion of poorer pupils achieving five GCSEs at grade C or above, including English and maths.
In 2014, 30.7 per cent of pupils eligible for free school meals achieved this. But in 2015 the figure fell to 23.8 per cent.
In a letter to Reading Borough Council, Bradley Simmons, the Ofsted’s regional director for the South East, said the drop was "unacceptable".
But the council said that only two of the area's eight secondary schools were under local authority control. Reading has six academies of which two are grammars, and two council-maintained schools.
However, Mr Simmons warns of a “wide and worsening gap” between the performance of the most disadvantaged pupils and that of their peers.
He said Reading schools’ overall performance was “not strong” and that the total proportion of pupils scoring five C grades or above including English and maths had fallen from 59.3 per cent in 2014 to 57.5 per cent in 2015.
This was “of concern”, he said, but it was "completely overshadowed by the shocking fall in the GCSE performance of pupils eligible for free school meals".
He acknowledged the council was not solely responsible for improving schools, but he added: "Much more needs to be done to increase the level of challenge and support for secondary schools and academies in the area...
"I trust you will recognise that, whether pupils attend an academy or a maintained school, the local authority has a key responsibility to act as a champion for its most vulnerable pupils."
Mr Simmons added in a statement: "Pupils on free school meals in Reading get the worst GCSE results of any area in the South East.
"I am concerned about overall GCSE results in the borough. These pupils deserve better if they are to have a brighter future...that is why I am calling on Reading Borough Council to explain what they are doing to address this serious situation."
Tony Jones, Reading’s lead councillor for education, said: “As [Mr] Simmons will know from his time in the south west, it took Bristol 10 years to move from the bottom to 21st in the country. We aim, and have begun to demonstrate with the marked progress in primary schools, to make the same jump in improvement but in a much shorter timescale.
"The Ofsted letter further strengthens the council's decision to establish a school-led improvement partnership which seeks to improve schools across the town to good or outstanding by 2018 and to significantly raise and close the gap in attainment for all at the same time."