A Coastguard search was under way today amid fears that a ferry passenger had fallen into the sea.
The man is understood to have boarded the boat at Larne in Northern Ireland but he was reported missing after it docked at Cairnryan in Scotland.
Coastguard teams said they were "looking into all scenarios".
The Maritime and Coastguard Agency said the European Highlander ferry left Larne at 7.24am and arrived at Cairnryan two hours later.
The man was reported missing by a coach driver after the vessel arrived in Scotland.
Clyde Coastguard was notified shortly after 10am and a full search of the ferry and surrounding area was organised, taking into account tidal drift and winds.
The Portpatrick, Stranraer and Larne RNLI lifeboats were all asked to launch and a Royal Navy rescue helicopter was scrambled from Prestwick.
The Maritime and Coastguard Agency said sea conditions were calm with a slight swell and good visibility.
A Clyde Coastguard spokeswoman said: "There is a search under way. We're also carrying out shore-side investigations to try and find out where this person is.
"It appears someone got on the ferry at Larne and didn't get off at the Cairnryan side, so we have to look at the possibility that there is a man overboard.
"We're looking into all scenarios at the moment. It hasn't been confirmed that there is a person in the water.
"We're still carrying out other investigations while we're doing the search."