Four million 'at risk of flooding which could kill hundreds and spark riots' ... in BRITAIN

More than four million Britons should be warned they could lose their lives, homes or possessions if a nearby dam or reservoir bursts, the official inquiry into last summer's floods said yesterday.

The breach of just one reservoir could kill hundreds, swamp thousands of homes, rip apart key gas mains and wipe out electricity for an entire city.

Yet the maps that could reveal which properties are at greatest risk are kept secret because of fears they could encourage terrorists, the inquiry found.

Sir Michael Pitt, who led the report, said 'urgent and fundamental' changes were needed to improve flood defences.

He demanded fresh building regulations to make new homes flood-proof, and called on the Government to take flood risk as seriously as terrorism.

The Pitt Review was published a year to the day after the start of last summer's floods  -  Britain's biggest peacetime emergency since the Second World War.

Torrential freak storms swamped 55,000 homes, killed at least 13 and left half a million without mains water or electricity.

Insurance claims totalled £3billion.

Yorkshire, the Midlands and the Thames Valley were worst hit.

Sir Michael said: 'It is unacceptable that one year on, thousands of people remain in temporary accommodation.

'Research shows that the risk of flooding continues to escalate, making the events that shattered so many communities last year an ever increasing threat.'

Caravans outside a home in Hull, where families are still unable to move back into their homes following flooding last year

The review said the disaster could have been even worse. Expert say there was a 50-50 chance that a dam at Ulley in South Yorkshire holding back more than 100million gallons of water could have breached.

A 60ft gash was spotted in the Victorian dam, causing around 1,000 people to be evacuated and part of the M1 closed.

Roger Hargreaves, head of the Pitt Review team, said: 'The water would have flooded homes, ripped through a major north-south gas main that supplies Yorkshire and shut down an electricity switching station that supplies 750,000 people in Sheffield.

'The gas could have interfered with aircraft flying overhead.'

Flood advice

Of the 2,000 reservoirs in Britain, 1,000 pose a risk to life if they breach.

But concerns about dams being terrorism targets mean few know their homes are potentially at risk.

The Pitt Review said maps showing these inundation areas should be made public.

Water and electricity companies should do more to share information about their key, vulnerable sites.

Sir Michael said new homes, or those refurbished in flood areas, should have proper drainage and flood gates, concrete floors and electricity sockets high up on walls.

He called for a new Cabinet Committee dedicated to tackling floods, and for an Environment Agency and Met Office nerve centre to issue better flood warnings.

Electronic maps showing every drain in Britain should be created, while councils should take greater responsibility for flash flooding.

The report also said emergency services wasted too much time handing out ineffectual sandbags and called for developers to stop building on flood plains unless 'absolutely necessary'.

It called on the Environment Agency to start an 'opt-out' phone warning system, with at-risk householders automatically signed up for alerts.

Home information packs should have details on flood risk. People should take responsibility for protecting themselves and possess a flood kit including torches, rubber gloves and documents.

Environment Secretary Hilary Benn said the Government would publish a detailed response to the report in the autumn.

Tory environment spokesman Peter Ainsworth said: 'The report calls for "strong national leadership" yet all we have had is more dithering from the Government.

'Pitt's interim report described last year's floods as "a wake-up call", but someone seems to have hit the snooze button.'