A new free school due to open in September has only just received funding, despite already hiring teaching staff and offering school places.
Scheduled to welcome a reception class in less than two months, Park Community School's funding from the Department of Education (DofE) was only confirmed last week.
Education charity Chapel St, which runs the school, has already come under fire from councillors for failing to secure a site before the first round of reception class allocations in April.
The primary school, which will have one class for 30 children in its first year, will open from a temporary site in Crown Lane, Morden, because a permanent site in Tyndale House, Dorset Road, is not yet ready.
Now Chapel St has admitted the school allocated places before crucial funding was confirmed.
A spokeswoman from Chapel St said: "There are different options for schools that don't receive a funding agreement, such as opening the school at a later date, but we're thrilled that we don't have to consider any of these options."
But a DofE spokesman said before funding is confirmed, "they can't go ahead and open".
Councillor Martin Whelton, cabinet member for education, said: "It's a shame there has been so much uncertainty given the pressing need for school places in Merton.
"I think it is extremely regrettable that this has been a last-minute announcement and shows the need in terms of proper planning for school places."
The Secretary of State for Education confirmed funding for the school on Wednesday, July 23.
Russell Rook, chief executive of Chapel St, said: "As a parent myself, I’m very aware of the importance of choosing a good performing school where my children will thrive.
"I’d like to thank those who have supported the opening of the new school for their determination to see a much needed additional primary school in Merton; one in which we can have confidence and pride."
Chapel St has since applied to open a secondary school, Trinity High School, in Merton in September 2016.