Houses on stilts, floating communities and returning urban areas to saltmarsh should all be considered as part of radical approaches to tackling the threat posed to coastal cities by flooding, a report suggested today.

The study by the Royal Institute of British Architects' think tank Building Futures and the Institution of Civil Engineers warned that the UK's 12,000km (7,500 miles) of coastline would be increasingly at risk of floods.

Rising sea levels, sinking landmass in the south and increased storminess will all put coastal towns and cities at risk, and planners will need to decide how to tackle the threat.

At-risk areas could cope with the risk of flooding by retreating inland and allowing water to flood parts of the city, defending urban sites in innovative ways or "attacking" the problem, for example by building out into the water.

The report focuses on two at-risk cities, Hull and Portsmouth, and lays out possible strategies for coping with flood risk up to 2100. The scenarios were masterminded by architects, civil engineers, planners and city designers, developers, policy-makers, ecologists and "futurologists".

According to the "retreating" scenario, Hull, which is mostly only 2m to 4m above sea level but is an important port, could see the historic centre protected and turned into an island linked to the rest of city by bridges.

Much of the urban area alongside the river could become saltmarsh, which provides a natural defence against flooding, while new developments could be shifted to higher ground and a relocated train station built on stilts.

Or the city could be defended with a sea wall made up of a series of walled reservoirs, backed by a number of creeks to relieve pressure on water systems, and used for grey water storage, reedbeds, fish farms and even recreation.

A more attacking strategy could see a network of platforms along the coast, perhaps using recycled oil platforms, for a mixture of homes, businesses and leisure centres, with energy provided by tidal stream generators on the seabed.

Buildings in the city could be retrofitted to cope with flooding while low-lying land could be changed from residential to recreational areas.

Portsmouth is also an important city and harbour - particularly for the Navy - and also low-lying.

For the city to take up a strategy of "retreating", the M27 would have to be moved, while frontage could be returned to public beach and east Portsea Island turned into saltmarsh - providing the scope for the biggest urban saltmarsh sheep farm in the UK. Other development could include algae farming.

Or it could be defended with tide gates to protect against tidal surges and a new sea wall as a "living wall" with commercial, residential and recreational properties included to entice private investment in the defences.

Under the attacking strategy drawn up in the report, a series of inter-connected piers could extend from the land, which would have a two-tier design with transport underneath a pedestrian top layer.

The piers would contain residential, business and recreational development, and lead to the creation of marinas and floating communities as part of efforts to increase the amount of space in the city available to be developed.

Homes in vulnerable areas could be adapted so that living space was moved upstairs while new development could be on stilts or able to float in times of flooding.

RIBA president Ruth Reed said: "The scenarios we have created are extreme, but it is an extreme threat we are facing.

"Approximately 10 million people live in flood risk areas in England and Wales, with 2.6 million properties directly at risk of flooding from rivers or the sea.

"However, if we act now, we can adapt in such a way that will prevent mass disruption and allow coastal communities to continue to prosper. But the key word is 'now'."

Chairman of the ICE steering group, Ben Hamer, said: "A proactive and united, almost war-like approach is needed if we are to win the battle against what is set to be our biggest challenge in the next century, the 'water invasion'."

He said the UK needed to urgently change the way it planned, built and designed at-risk communities, and there would have to be some very creative thinking about how solutions could be developed to be financially sustainable.

Bret Davies, coastal engineer at Portsmouth City Council, said the report was designed to provoke discussion but the "retreat" and "attack" proposals were not realistic for the city.

"At the moment, we're working with other councils on producing a North Solent shoreline management plan. Residents can have a say on this now, before it's finalised.

"The plan proposes defending the Portsmouth coastline for the next 100 years by 'holding the line' - which means improving and maintaining the sea defences we have now.

"We believe this is the best long-term policy for Portsmouth, and an affordable one."