Househunters must check out the current and future flood risk of the property they are buying to avoid paying through the nose for insurance premiums, insurer More Than warns.

Figures from the Environment Agency show that the number of homes at risk of flooding could increase from 1.5 million to 3.5 million as the effects of climate change take their toll.

The cost of flood damage could consequently rise from £950 million a year to £3.2 billion, according to environmentalists. It has risen by 60 per cent since 1998.

But More Than claims that there could be a further rise in homes affected by flood damage if plans to build at least 200,000 new properties in high risk areas are given the go ahead.

Many areas still face the threat of rising waters despite defences being built in the most high-risk locations. The government has also promised to build defences for a further 80,000 properties but those living in the countryside could still find themselves left to sort out their own flood protection.

Although most insurers offer flood cover, a new agreement by the Association of British Insurers (ABI) says that defences must be in place by 2007 to guarantee cover for houses expected to flood more than once in 75 years.

Homes without cover stand to lose up to 80 per cent of their value.

Some insurers have also come up with their own maps to assess flood risk. Richard Mason of Insuresupermarket said: "Switching to an insurer that has adopted a more sophisticated method of assessing risk could save you more than £100 a year. But it might also show you to be a higher risk."