Ealing Fields High School will not open in September as planned, as negotiations over the preferred site could not be completed in time.
The Department for Education (DfE) has now deferred the free school’s opening until September 2016.
Dr Judith Mortell, leader of the school's steering group, described the news as "devastating" for parents but said: "We did everything within our power to avoid it."
She added: "We have completed all the required aspects of opening that were in our control and we pushed the EFA (Education Funding Agency, the funding wing of the DfE) as hard as we could regarding site acquisition.
"Whilst negotiations remain at a tantalisingly close stage, there is no longer sufficient time to complete the deal and undertake the necessary work that would be required to create an appropriate learning environment in which to welcome our first cohort of students this September."
Ms Mortell said parents’ applications to other high schools had not been affected by the DfE’s decision.
She added: "We have worked closely with the local authority. In line with their normal admissions procedures, the council has made sure that families will be offered a place at the school they would have been offered, had they not applied to Ealing Fields High School."
Ealing Fields High School executive headteacher, Dr Mark Fenton, and headteacher, Lee Faith, remain in post for opening in September 2016.
Mr Faith said: "The decision to defer our opening is of course, deeply disappointing for all stakeholders in our community - from year six children and their parents/carers to the members of the trust who have worked tirelessly to ensure we would open in September.
"My job now is to keep working hard to maintain the high levels of support we enjoy from the community and to make sure we can deliver our educational vision for young people in south Ealing in 2016.
"This news is a major blow to families in south Ealing. In the face of this disappointment we will redouble our efforts to make sure that there is no repeat next year."
Ealing Fields High School will be holding drop-in sessions over the coming months to update parents about progress towards opening next year. Dates and venues are yet to be announced.