The King said he is "profoundly shocked and saddened" in a moving message to King Felipe following the train crash in southern Spain.
At least 41 people have been killed following a two-train crash in Adamuz, Andalusia on Sunday evening.
In the statement released by Buckingham Palace on Tuesday, Charles, 77, wrote to the Spanish monarch: "My wife and I have been profoundly shocked and saddened to learn of the appalling train accident in Southern Spain and of the many lives so tragically lost in such a disaster.
"We offer our deepest possible sympathy to all those who have suffered the heartbreaking loss of their loved ones and our special thoughts remain with those who have been terribly injured in this dreadful accident."
He continued: "We also wish to pay a particular tribute to the emergency services who responded so swiftly and courageously at the scene of the crash.
"We have the greatest admiration for their selfless service and unwavering commitment to helping others in their time of need.
"We hold the people of Spain in our most heartfelt prayers and stand with you in deep sorrow at this painfully difficult time."
© Europa Press via Getty ImagesKing Felipe, 57, and his wife, Queen Letizia, 53, postponed their engagements to travel to the Reina Sofia University Hospital in Cordoba to visit the injured and their families on Tuesday.
The royal couple also expressed their gratitude to emergency responders and local residents and volunteers who provided initial assistance to the victims.
What happened?
The crash took place on Sunday at 7.45pm when the tail end of a train carrying 289 passengers on the route from Malaga to Madrid, went off the rails. It collided with an oncoming train travelling from Madrid to Huelva, according to rail operator Adif.
The head of the second train, which was carrying nearly 200 passengers, took the brunt of the impact. That collision knocked its first two carriages off the track and sent them plummeting down a 4m (13ft) slope.
© Europa Press via Getty ImagesOfficials are continuing to investigate the causes of the incident that Spanish transport minister Oscar Puente Santiago has called "truly strange" since it occurred on a straight line and neither train was speeding.
But he said in interviews with Spanish media that officials had found a broken section of track that could possibly be related to the accident’s origin, while insisting that is just a hypothesis and that it could take weeks to reach any conclusions.
"Now we have to determine if that is a cause or a consequence (of the derailment)," Mr Puente told Spanish radio Cadena Ser.
© Europa Press via Getty ImagesBoth Mr Puente and Alvaro Fernandez, the president of Renfe, said that both trains were travelling well under the speed limit of 155mph and "human error could be ruled out".
Spanish prime minister Pedro Sanchez visited the accident site near the town of Adamuz on Monday, where he declared three days of mourning with flags lowered on all public buildings and navy vessels.
Additional reporting by PA.
