The cost of outstanding work to boost Sheffield's flood defences has soared by nearly half to £93 million.
That is the expected bill for flood prevention measures along the Upper Don, Sheaf and Porter Brook rivers.
The total cost of new flood defences was previously estimated at £83m, including the £19m already spent protecting businesses along the Lower Don Valley, where work is nearing completion.
That means the anticipated bill for the remaining improvements has increased by £29m from the £64m initially forecast by Sheffield Council.
Although £22.5m of government money has been earmarked for the work, subject to approval, that now leaves a £70.5m funding shortfall.
However, the council remains confident it can secure that money and says it is already in talks with potential investors, including Sheffield City Region.
It says the latest plans would provide even greater protection than those originally envisaged, safeguarding the city against flooding so severe it would only be expected once every 200 years, with extra resilience built in to cope with the more severe storms expected due to climate change.
They would also be less harmful to the city's ecology, the council claims, following concerns the original proposals could damage wildlife habitats including ancient woodland.
James Fletcher, the council's flood and water manager, said the latest plans would not only protect the city from flooding but could drive down insurance bills for householders and businesses, and open up new land for development.
"While the opportunity is there, we want to ensure flood protection is in place for generations to come and to help build the economy in the process," he said.
"The Government is getting the picture now in terms of this being vital infrastructure for a core city that's an important part of the Northern Powerhouse.
"We're saying to the Government 'this is what it's going to cost, and now we have to work with you on how we're going to pay for it'."
The entire project, launched in the wake of the devastating 2007 flood, in which two people died, was initially due to be completed by 2021.
The council accepts this will no longer be possible, with detailed designs for the remaining work yet to be drawn up.
It has not set a new estimated finish date, but plans to carry out improvements in stages over a 'five year-plus' period as the funding is unlocked, beginning with the Upper Don.
The increased cost was revealed as the council announced its preferred options for new flood defence measures, following public consultation.
They include plans for 'flood storage areas' at Endcliffe Park and Abbey Brook, which would hold water temporarily during extreme downpours.
The council now hopes to work with Yorkshire Water so existing reservoirs can be used to hold floodwater, meaning controversial proposals for a dam on the Rivelin Valley at Roscoe would not be needed.
Other measures on the table include new and strengthened walls, and the removal of 'pinch points' to speed up the flow of water through the city.
New flood defences are also planned at Blackburn Brook, running through Chapeltown and Ecclesfield, and at the 'Three Brooks', in Manor, where work is scheduled to commence next year.