The principal of a Norwich academy which has been issued a warning letter by the government about its 'unacceptably low' performance has insisted a comprehensive action plan has been put in place to make improvements.

City Academy Norwich is one of 33 academies throughout the country to have been sent a so called pre-warning notice from the Department for Education (DfE) to inform bosses about the poor standards of performance and warn them that a formal warning notice might be issued if matters are not addressed.

A formal warning, like that which has been issued to 17 academies in the country since the start of this term, calls for the schools to improve or face intervention which includes the possibility of being put under new management unless they improve.

The DfE notice came after the City Academy's performance against the headline league table measure of 5 A*-C GCSEs including English and Maths was brought down in particular by specific issues in one key subject area, English.

But bosses at City Academy Norwich, which opened in the former Earlham High in September 2009, say they have already moved to reassure the DfE that a 'comprehensive Action Plan to make improvements – in particular in GCSE English – following a disappointing set of exam results this summer' has been formally put in place after being sent the warning.

David Brunton, principal, said: 'Our results were significantly affected this year by a one-off set of circumstances, in one particular curriculum area, which we have acted swiftly to address.

'Working closely with our Governors, the TEN Group and outside specialists, we have in place an action plan that is already yielding marked improvements in our students' performance in English.

'With 100% of our Year 11s last year progressing on to further education or employment, improving maths results, and some outstanding individual results at GCSE, the foundations are in place for a much improved performance this year and beyond.'

City Academy Norwich has already implemented most of the key points contained within the Action Plan. The actions taken have included bringing in new English teachers, appointing an outside specialist to assist with our literacy strategy, utilising the expertise of the recently-appointed TEN Group strategic lead for English and the setting up of a strong educational sub-group of Governors to oversee the monitoring of the action plan, led by the Headmaster of Norwich School, our educational partner.

Other measures in the Action Plan centre around continuing recent improvements in maths, raising overall attainment, and developing the use of data and curriculum review to support further improvement.

Pre-warning letters, as they are called, are part of the process by which action is taken on sponsored academies which are not improving as well as they should. The standards of education they are providing are considered to be unacceptably low.

The letters, all from junior education minister Lord Nash and his predecessor Lord Hill, appeared on a government website alongside 17 other older pre-warning and warning letters requiring academies to improve.

The letter to City Academy Norwich stated that standards of performance of pupils at the academy are unacceptably low because:

• Attainment for pupils is below the minimum floor standard, with just 22pc of pupils achieving a 5+ A*-C GCSE (or equivalent) including English and maths.

• Not enough pupils make or exceed expected levels of progress, with only Attainment for pupils is below the minimum floor standard, with only 41pc of pupils making expected levels of progress in English, and 51pc making expected levels of progress in maths.

• Attainment dropped from 40pc of pupils achieving 5+ A*- C GCSEs including English and maths 2012, to 22% achieving that level in the Academy in

2013, against a prediction of 46pc.

The letter from Lord Nash went on: 'The Secretary of State and I are therefore satisfied that the standards of performance at the City Academy Norwich are unacceptably low.'

It added that the academy should respond in writing 'outlining the actions that you will be taking to remedy the matters identified.'

A DfE spokesman said: 'The majority of sponsored academies are thriving under great leadership. Their GCSE results are improving far faster than in other state-funded secondary schools.

'This is despite sponsored academies taking over from schools that were consistently under performing, which can take time to reverse.

'But results in a minority of sponsored academies remain stubbornly low. We will not tolerate long-term under performance in any school - including in an academy.

'That is why this government issues pre-warning letters and warning notices. The evidence shows that academies respond well to these warnings, achieving on average much better GCSE results afterwards.'

The announcement comes at a time when the education system across the whole of Norfolk has come under intense scrutiny. In June Lisa Christensen, director of children's services, stepped down following criticism of her department by Ofsted and pressure from MPs.