A nine-kilometre stretch of cycle path given planning permission now faces a race against time to be completed.
The north-south cycle route would link the Newhaven, Peacehaven and Seaford with Lewes and beyond along the west side of the River Ouse.
The application was approved by the South Downs National Park Authority (SDNPA) in July but the money to build it must be claimed by March or funding from the Government, worth £600,000, will be withdrawn.
The new path, phase three, follows phase one from Kingston to Lewes and phase two from Rodmell to Southease.
The Ouse Valley Cycle Network (OVCN), a voluntary group, helped get the route to this stage.
Its big hurdle is getting permission from all the landowners – a mixture of public bodies and private individuals – to decide on an exact route.
The path will be used by walkers, cyclists and wheelchair users. Horses will also be allowed on some sections.
Rod Lambert is a committee member of the Cycle Seahaven club.
He said: “The campaign has gathered momentum – especially with the creation of the South Downs National Park and its pro-cycling stance.
“This is a fantastic opportunity for cyclists to gain a safe, picturesque route from the Sussex coast northwards linking Newhaven and Peacehaven with the neighbouring towns and villages, and provide a safe and functional route for leisure, tourism, commuters and students alike.
“It would be a beautiful addition to the Avenue Verte and could perhaps help turn Newhaven into the Cycling gateway to Europe and, in turn, help save the Newhaven to Dieppe ferry route from closure.”
Cycle Seahaven’s chairman Jamie Lynch said: “We need to grasp this opportunity to make a safe and healthy cycle route for future generations to enjoy.”
A SDNPA spokeswoman said: “We are working with our partners Sustrans and OVCN to ensure the scheme will be delivered within the timeframe set by the Department for Transport.”
She said negotiations with landowners were ongoing and progressing positively but could not say when agreements would be reached.