They own the boundary wall along the River Arun at Arundel, West Sussex, which gave way in January after high tides and heavy rainfall.
But their insurers have refused to pay for repairs and the Environment Agency said it is the owners' responsibility.
Local MP Nick Herbert described it as a "deeply unsatisfactory situation".
Johnny Boylan, who is one of three affected homeowners, said the wall collapsed eight months after he and his partner moved into their house on River Road.
He said they repaired the top section of the wall when they moved in but had not realised at the time they owned the full wall down to the river bed.
"There is no mention of the river wall in our deeds," he said.
"It's an ancient part of the wall and now our insurance company is saying we are not insured because it was neglected."
Mr Boylan and his partner are currently unable to live in the property.
"It's a deeply unsatisfactory situation," Mr Herbert, MP for Arundel and South Downs, said.
"The Environment Agency and the public authorities are saying this is the householders' responsibility.
"That would be fine except that the insurers are saying this is the Environment Agency's responsibility."
He added the "poor policyholders" were now in a position where they may have to meet the bill "of several hundred thousand pounds".
The Environment Agency said its last inspection in September 2015 had not highlighted any "major issues" along the wall.
It said affected homeowners were "co-ordinating how repairs to this private flood defence will be taken forward, and we continue to advise them as appropriate".
Residents who had to evacuate their homes after a river wall collapsed are locked in a dispute over repairs they fear could cost £500,000.