A £120m scheme to build new sea defences in Southsea is facing a shortfall of more than £24m.
Portsmouth City Council wants to renew the ageing defences between Old Portsmouth and Eastney.
The project is expected to protect more than 8,000 properties from flooding.
The council wants central government to make up the shortfall. The Department for Environment, Food & Rural Affairs (Defra) said it was working with the council to find additional funding.
The government has already agreed to contribute £90m to the scheme and the city council is putting in £6m.
If the extra £24m cannot be found, the city council said the scheme would have to be scaled back.
Steve Pitt, of Portsmouth City Council, said: "We need central government to step up, especially because we've got historic monuments here that need to be protected, including Southsea Castle where Henry VIII stood and watched the Mary Rose sink.
"This is not small stuff, this is major tourism issues for the city."
A spokesman for Defra said: "The government and Environment Agency continue to actively support Portsmouth City Council to find sources for the additional funding they need for this project."
He added Defra was already investing "a record" £2.6bn to support more than 1,500 flood defence schemes around the country aimed at protecting 300,000 homes by 2021.
It is hoped work in Portsmouth will start next year and end in 2026.
The council said the defences on the 2.8-mile (4.5km) stretch of seafront, some of which are 100 years old, were likely to fail within 10 years.
Temporary repairs have been made to plug holes and shore up several collapses in sections of the sea wall.
Work could feature a combination of higher vertical sea walls and some sloped stepped areas with wider promenades, the council said.