HUNDREDS of householders throughout North and East Yorkshire must have their homes registered as being at official risk of flooding, it emerged today.

Warning letters will be sent to affected residents - who may then have to revise home insurance policies and advise potential buyers if they sell their homes.

Up to 700 homes in York will be among the first in the region to be told they are officially at risk of flooding - even though they are protected by a £10m flood defence system.

Properties near other rivers - including the Derwent, Nidd, Swale, Ure, Wharfe and Tees - will also be put on notice of the risk assessment by the Environment Agency.

No specific details are currently available of the homes affected outside York, although it is known that homes in Malton, Norton and Stamford Bridge - where major floods wreaked havoc last year - will be warned.

The agency will be alerting 39,000 properties, from Hartlepool to Howden and Goole, which are within the "indicative flood plain".

The notices, which householders should receive by the end of July, are being issued under new Government regulations following catastrophic Easter floods that hit the Midlands in 1998.

Experts stress the threat to York homes - all in areas protected by flood defences such as low Bootham, parts of Clifton Green, parts of Tang Hall and large areas bordering the Foss - is extremely small.

But properties could be inundated if water levels rose higher than flood defences could cope with or if defences were breached in some way.

"There can be no absolute guarantee that they would never be flooded," admitted Environment Agency flood defence engineer Bob Parry. "All we can do is reduce the risk by putting in defences."

The shake-up will see new flood warning systems introduced across the country - with the old yellow, amber and red alerts being scrapped.

And the Agency has given itself until September to formally notify homeowners in a flood risk area in York that they are at risk.

Robert Seaton, head of residential property at York solicitors Harrowell Shaftoe, said that under existing home-selling rules, home-owners were likely to have to declare the "new" flood risk to prospective buyers.

Affected homeowners could see their insurance premiums rise and Mr Seaton urged them to check their home insurance immediately.

Ben Hudson, partner in estate agents Hudson Moody, said: "A flood risk will certainly have an effect on the value, and vendors would have to disclose it."

But, he said, buyers were likely to be less worried about a property which was last flooded a long time ago, say 50 years.

John Simmons, City of York Council's emergency planning co-ordinator, said in a worst-case scenario, the number of homes in the York area protected by flood defences which could be inundated was up to 700.

Mr Parry confirmed today the process of notifying "at risk" homes was about to begin.

He confirmed there could be up to 700 homes at risk - but stressed the risk was "extremely small".