On Wednesday afternoon, playback for Tuesday’s broadcast between 1.58pm and 5pm was unavailable on the station’s website.
Station manager Peter Moore said: “Due to a computer error at our main studio, the death of a Monarch procedure, which all UK stations hold in readiness while hoping not to require, was accidentally activated on Tuesday afternoon (May 19), mistakenly announcing that HM the King had passed away.
“Radio Caroline then fell silent as would be required, which alerted us to restore programming and issue an on-air apology.
“Caroline has been pleased to broadcast Her Majesty the Queen’s, and now the King’s, Christmas message and we hope to do so for many years to come.
“We apologise to HM the King and to our listeners for any distress caused.”
The blunder comes as the King and Queen visit Northern Ireland, where they joined a performance with a folk group.
Charles and Camilla also watched dancers and sipped Irish whiskey in Belfast’s Titanic Quarter on the first day of their trip.
Established in 1964, Radio Caroline is a former pirate radio station based in Maldon, according to its website and social media.