The government decided to approve an application for a 1,710-place free school despite there being more than 1,600 surplus places within 2 miles of the proposed site, official documents show.
However, the head of the free school has said the numbers used by the Department for Education were "historic" and the extra capacity was needed for an "expanding town".
The free school impact assessment, published for the first time today, acknowledged that approving plans for Great Western Academy in Swindon would have a "high" impact on three of the closest secondaries, and a "moderate" impact on a fourth.
The existing four schools already had 503, 220, 458 and 441 spare places respectively in May 2017, the Department for Education found. More than half these places (899) were in two schools rated "good" by Ofsted.
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The approved application for Great Western Academy states that it will have a capacity of 1,710 by 2022 across its secondary school and sixth form by 2022.
But principal Graham Davis said this has since been reduced to a maximum capacity of 1,210 places.
He said: "I realise there are some free schools which have been opened where maybe they should not have been, but we are not one of them.
"Swindon is an expanding town. Great Western Academy is built to serve a new estate of over 1,800 houses.
"We are full and oversubscribed in Years 7 and 8, the other schools you mention are also full in Year 7 due to the increasing numbers in the town, and we have very positive working relationships with them to help promote the excellent education available to the new population of North Swindon.
"We are not a threat to them in any way – in fact, the exact opposite I would suggest.
"The excess capacity in the other schools is historic. It is not there in lower year groups, where there is a significant local demographic increase in students of age 13 and below."
He also said the decision was made based on "known numbers" of pupils at local primary schools, which would influence future secondary cohorts.
The new free school opened in September 2018 and is yet to be inspected by Ofsted.
A DfE spokesperson said: "The system for approving new free schools is underpinned by a rigorous application and evaluation process.
"We will only open a free school when we are satisfied the school will deliver a good standard of education and be good and viable from day one."