A map showing how a "retreat" strategy would change Portsmouth
Rising sea levels and more storms could mean that parts of at-risk cities will need to be surrendered to protect homes and businesses, a report warns.
The authors say that "radical thinking" is needed to develop sea defences that can cope with the future threats.
About 10 million people in England and Wales live in flood risk areas.
The project, launched on Friday, is a joint venture between the Institution of Civil Engineers (Ice) and the Royal Institute of British Architects (Riba).
The report, Facing up to Rising Sea Levels, urges the government, planning authorities and the public, to act sooner rather than later.
"If we act now, we can adapt in such a way that will prevent mass disruption and allow coastal communities to continue to prosper," said Riba president Ruth Reed.
"But the key word is 'now'," she added.
The study warns that rising sea levels, an increase in the frequency of storms and sinking landmasses could leave many UK coastal areas vulnerable to extreme flooding.
Facing the future
The project focuses on Kingston-upon-Hull and Portsmouth, two cities deemed among the most at-risk areas in the UK, and presents a series of six scenarios set up to 90 years in the future.
The scenarios are based on three themes:
• Retreat - moving "critical infrastructure" and housing to safer ground, allowing the water into parts of the city
• Defend - building city-wide sea defences to ensure water does not enter the existing urban area
• Attack - extending the existing coastline and building out on to the water (using stilts, floating structures and/or land reclamation)
The scenarios were developed by a group of experts, including architects, civil engineers, city designers, developers and policymakers.
Their ideas also had to take into account a number of constraints, including a lack of funding to protect the entire coastline of the UK, which stretches for more than 12,500km (7,700 miles).
"The scenarios we have created are extreme, but it is an extreme threat we are facing," observed Ms Reed.
The authors decided to focus on Kingston-upon-Hull and Portsmouth because the two cities displayed many of the characteristics of flood-prone areas.
They said that Kingston-upon-Hull was a low-lying city that has to be constantly drained by pumps, the River Hull - which runs through the middle of the area - is liable to flooding, and the port is of national strategic importance.
Portsmouth was also vulnerable, they added, because most of the city is located on an island that was no more than 3m (10ft) above sea level, and was densely populated.
Ben Hamer, chairman of the Ice steering group, said that government, public officials and businesses needed to work together in order to halt the "water invasion".
"Some very difficult decisions need to be made in the future, and to do this we need integrated thinking," he said.
"The UK must urgently change the way it plans, builds and designs at-risk communities."
The project's findings, which include sketches and details of the proposed "new cities", will be presented in a exhibition that will visit London, Portsmouth and Kingston-upon-Hull.