Three crew members are missing after two boats collided and one sank off the West Sussex coast, the RNLI says.
The boats collided about two miles off the coast of Shoreham. A fourth person was rescued and taken to hospital.
The RNLI is carrying out searches between Worthing pier in the west and Brighton in the east.
The coastguard said the search began after a man was found in the water about two miles off shore at about 05:50 BST.
HM Coastguard’s search and rescue helicopters from Lydd and Lee-on-Solent, together with coastguard rescue teams from Shoreham and Littlehampton and RNLI lifeboats from Shoreham and Brighton as well as several other local vessels are involved in the search.
Other people believed to have also been on board a small vessel may be in the water, a coastguard spokeswoman said.
The rescued man was discovered by a fishing boat.
Andy Jenkins, controller with the UK Coastguard, said: “The information we have received says there are potentially three others who are in the water. We are carrying out an extensive search for them.”