An air and sea search is under way for a person believed to be missing in the water after a boat capsized.
The Coastguard said the alarm was raised after two people were found holding on to the side of an overturned boat in Poole Harbour, Dorset.
The men were rescued by helicopter and taken to Poole Hospital.
Lifeboats are now working alongside the Coastguard helicopters in an "intensive" search for a third person believed to have been on board.
A spokesman for the Coastguard said: "We are in the middle of a major incident.
"Two people have been recovered from the water in Poole Harbour by coastguard rescue helicopter and transferred to hospital.
"An intensive search is ongoing for a third person believed to be missing in the water after a small boat capsized this morning."
The boat capsized near Brownsea Island and members of the public on the shore dialled 999.
Dom Fraser at the North Haven Yacht Club at Sandbanks told the Bournemouth Echo he understood the three people had been in a dinghy.
He said: "Around 10.45 I heard people shouting for help. I was about to row over but then the helicopter arrived.
"The water is quite shallow where it happened, to the point where you can stand up."
Volunteers are searching the shoreline and a number of private boats are believed to be helping the Coastguard.
The Coastguard spokesman added: "Portland Coastguard received 999 calls shortly before 11am reporting that there were people in the water from a small capsized boat in the Whitley Lake area just inside the harbour entrance.
"Both coastguard rescue helicopters were on exercise nearby and were on scene quickly.
"RNLI lifeboats from Poole and Swanage are searching inside and outside the harbour and Swanage and Poole Coastguard Rescue Teams are searching the shoreline.
"A coastguard rescue helicopter also remains on scene searching."
The search for the missing man has been called off following an "intensive three hour search" of the harbour, the Coastguard said.
Portland Coastguard co-ordinated the air and sea search but the man was not found.
A spokesman said: "A marker buoy placed in the location of the capsized boat indicated that a casualty on the surface of the water would have remained within the harbour area in the tidal and weather conditions during the search."
The search involved two coastguard rescue helicopters, RNLI lifeboats from Poole and Swanage, Coastguard rescue teams and local vessels as well as other members of the emergency services and the community.