A new free school in Hounslow could affect the long-term future of Kingsley Academy, a new government report claims.
The Department for Education has assessed the potential impact of all free schools which opened last year , including Nishkam School West London, in Isleworth , on existing schools in the area.
Overall, it concludes the Sikh free school, in London Road, which caters for pupils of all religions, will have a 'minimal' impact on established schools nearby.
But it judges there will be a 'high' impact on Kingsley Academy School, just over a mile away in Hounslow town centre.
"The free school (Nishkam) is likely to affect the long-term financial viability of the school (Kingsley Academy)," it states.
The judgement is based on the fact Hounslow Manor, as the secondary school was then known, had surplus places in 2011 and 2012 and had below average levels of attainment.
However, the report, which does not include subscription rates for this academic year, acknowledges that Kingsley Academy is rated 'good' by Ofsted.
It also points out that Nishkam, which currently caters only for primary age students, is not due to begin accepting secondary age pupils until September next year, and will not reach its full capacity of 1,400 until 2020.
Responding to the government report, Kingsley Academy headteacher Anjali Sakhardande said the school was in the top two per cent in the country between 2011 and 2012 for pupil progress and had been fast-tracked under the Government's Priority School Building Project to get a new building worth more than £16 million.
"We will be admitting our full complement of year seven students in September 2016 and they will be getting a brand new building in time to meet the growing needs of our local community and primary schools," she added.
"Our improving results, state-of-the-art facilities and journey to excellence will ensure that we will meet the growing need for high quality secondary school places in the next few years."
The report also concludes Nishkam will have a 'moderate' impact on Marlborough and Worple primary schools, and Isleworth & Syon School for Boys, Gumley House RC Convent School and Brentford School for Girls.
A spokeswoman for Nishkam School West London said: "We are here to improve the life chances and academic success of all our pupils. We make no excuses for setting very high aspirations for all of our pupils irrelevant of background.
"Any outstanding school will have an impact on its neighbouring schools, whether free school, academy or local authority-run, as it will provide a high level alternative and subsequent extension of choice for parents and pupils.
"This in turn will enrich the quality of educational provision available to all. However, most importantly, there is a sustained significant shortfall of pupil places in both primary and secondary phases projected in Hounslow within the next year going forward, which, even with Nishkam School's presence, will not be catered for until further new schools open."
