High tides and stormy seas have washed away aggregate intended to shore up a cliff face weakened by the recent heavy rain.
The pile of small rocks had been dredged from the River Sid by the Environment Agency and at East Devon District Council's (EDDC) request was diverted from its intended destination to Port Royal.
It was placed on the beach at Sidmouth to be transferred to Pennington Point in the New Year. A specialist machine, not available until January was to have moved the aggregate to the foot of the cliffs to protect it from wave action.
Councillor Phil Twiss, deputy cabinet member for environment at EDDC said that on Thursday night the material was taken by the seas.
"Sadly, our fear that this material might be washed away in the event of a storm has come true only too soon. It's very disappointing," he said.
"When we arranged for the material to be diverted to Port Royal, we knew there was a chance it could be washed away in a storm.
"We said at the time that it was a calculated risk that we needed to take.
"Unfortunately, nature has had the last laugh this time, but I don't regret giving it a go as it could so easily have been a short-term remedy".
EDDC is now in talks about the longer term measures to tackle cliff erosion.