Shoreham footbridge, which was due to reopen next week, will remain closed to the general public.
The northern end of the old bridge was removed last week ahead of the construction of the abutment for the new bridge.
But West Sussex County Council engineers have subsequently assessed the condition of the existing footbridge and concluded it is not safe for public use.
County Council Cabinet Member for Highways and Transport, Pieter Montyn, said: “The county C=council appreciates the inconvenience this will cause.
“However, despite the best efforts of everyone concerned to keep the footbridge open, it is in a worse condition than was expected.
“In the interest of public safety the footbridge will now remain closed until it can be fully demolished.”
The county council’s contractor Osborne will have limited use of the footbridge during the construction project for works access.
He added: “The shuttle bus service will be maintained throughout the closure period and the County Council has sought an improved service for those with mobility scooters and electric wheelchairs who are unable to access the shuttle bus service.”
The replacement bridge is still on track to open in summer 2013.
The marine piling has been completed with all seven piers now constructed.
BT Openreach has also completed the permanent cable diversions under the River Adur and removed the temporary cables from the footbridge.
Osborne will shortly be installing the sheet piled tidal wall adjacent to Lower Beach Road, as well as the first pre-cast concrete cross heads that sit on top of the piers.
Funding for the £9.8 million project has come from the County Council and sustainable transport charity Sustrans, which received a £50 million grant from the Big Lottery Fund to extend the National Cycle Network into towns and cities across the UK.
Sustrans Area Manager Nick Farthing said: “It’s disappointing that the current link has to close before the replacement bridge is open for use, but we know that Shoreham residents are looking to their fantastic new crossing, which will make it so much easier and safer to make local journeys on foot or by bike.”