Plans to build the new Cardiff bus station have "significant funding challenges" and face further delay, a report to the council has said.
Developers said the project, part of the £20m Central Square redevelopment next to Cardiff Central train station, cannot go ahead in its current form.
Rightacres' plans include student accommodation, shops, private flats, a car park and office space.
It now wants to change the scheme Cardiff council approved in March.
A report to be presented to Cardiff's cabinet next Thursday said Rightacres want to change the private flats to more student accommodation.
It also said if no tenants can be found for the offices, that should also be replaced with more student accommodation.
The bus station was initially due to open in December 2017 but the council warn any changes will require a new planning application.
"The current development proposal is conditional on securing sufficient tenants to fill the commercial space, " said the report by Cardiff council's economic development director Neil Hanratty.
"Although the developer is confident of finding tenants there is a risk of a delay to the development if sufficient tenants are not found to cross this threshold.
"If this delay were to materialise, this could result in the scheme needing to be redesigned which could have a detrimental impact on the delivery, timeliness and costs of the scheme."
The report, which emerged the day Cardiff council warned it faces a £74m budget cut over the next three years, added that the developer "will only commence development of the scheme once over 50% of the office area is let."
The old bus station was flattened for BBC Wales' new headquarters and the new one is planned for the old Wood Street car park site.
The report added: "The specific costs associated with the purchase the Wood Street NCP car park has placed a major burden on the bus interchange development."
The developers needed to "resolve" a number of "existing parking leases" before they demolished the car park meaning the new plans "needs to incorporate 225 car parking spaces".
But Rightacres said that would be "expensive to deliver and "dramatically increases the cost per space" making the "financial equation extremely difficult to resolve."
"It must be noted that the timing of delivery of a bus station facility remains dependent on securing appropriate tenants for the building," the report said.
The council has also made a bid to the Welsh Government for a contribution to the "technical fit-out costs" of the new bus station and improving the road around the new interchange.
Rightacres and Cardiff council said they would not comment on the uncertainty surrounding the bus station project.