Lord Smith said coastal erosion was his agency's most difficult issue
Some parts of the British coastline are so badly eroded they are not worth protecting from the sea, the new head of the Environment Agency has said.
Lord Smith of Finsbury said work was already under way to identify areas of the east and south coast most at risk.
He told the Independent that the UK faced hard choices over which coasts to defend and which to leave to the sea.
Lord Smith said parts of north-east Norfolk and Suffolk were in the most immediate danger of collapse.
The former culture secretary said it would not be possible to save all coastal homes from sea erosion, but that the agency would do its "level best" to protect places where there were significant numbers of properties.
"This is the most difficult issue we are going to face as an agency," he said.
"We know the sea is eating away at the coast in quite a number of places, primarily - but not totally exclusively - on the east and south coasts.
"It's a particularly huge issue in East Anglia, but in quite a number of other areas as well."
Lord Smith called on the government to assist families whose home will be lost, as ministers would not always be able to rely on insurance companies to cover them.
He added: "We are almost certainly not going to be able to defend absolutely every bit of coast. It would simply be an impossible task both in financial terms and engineering terms.
"We will publish next year details of the work that's been done, where we think the particular threats are, where we think there is current defence in place.
"We will begin to talk with communities where we think defence is not a viable option."
Lord Smith also said the government was not taking the environment seriously in several major proposals.
These included a third runway at Heathrow, a new generation of coal-fired power stations, and the Severn barrage tidal energy project.