A search and rescue operation was under way today after dozens of people taking part in a charity swimming race had to be pulled from the sea, the Maritime and Coastguard Agency (MCA) said.
One person remains unaccounted for after more than 130 swimmers got into difficulty off the coast of Southwold, in Suffolk, earlier today, an MCA spokeswoman said.
A second swimmer who was reported missing has since been found.
Four lifeboats and a rescue helicopter were involved in the operation after initial reports that up to 90 people were missing.
Two people were taken to Paget University Hospital in Great Yarmouth with suspected hypothermia, the spokeswoman added.
Coastguards have now urged the missing swimmer - a woman from the Kettering area - to contact them if she managed to leave the water safely.
An MCA spokeswoman said it was unclear how the swimmers got into trouble as weather conditions were fine but the water was "very cold".
Some swimmers were rescued by lifeboat while a number of people swam to the shore themselves, she said.
Everyone pulled from the water so far has been conscious and there have been no reported fatalities, the spokeswoman added.
"We're still trying to account for one woman who started the race," she said.
"If she has returned home without informing the coastguard, we are asking her to contact Humberside Coastguard on 01262 672317."
Eyewitnesses spoke of exhausted swimmers coming out of the sea and being wrapped in towels, while a lifeboat hut close to the pier has been turned into a makeshift emergency centre.
James Dawson told the Norwich Evening News: "It was really, really tough conditions and people were coming out of the water looking exhausted."
It was reported the event was the Southwold Pier to Pub Swim.
Organised in co-operation with Adnams of Southwold brewery, the swim was due to start at noon with the mile-long event ending with a visit to The Lord Nelson Pub.
The MCA said later that the final missing swimmer had been found and the search called off.
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