Emiliano Sala's former club Nantes have reacted with anger at the decision to formally call off the search for his missing plane and demanded it be resumed.
The French club's call came as the Argentinian striker's sister begged rescue workers to keep looking for the plane.
An emotional Romina Sala, speaking in Spanish, told reporters in Cardiff that she feels her brother and his pilot are still alive.
Ms Sala travelled to South Wales after a light aircraft carrying her brother and pilot Dave Ibbotson disappeared from radar on Monday night.
Guernsey's harbour master Captain David Barker earlier confirmed the "difficult" decision to end the search, saying the chances of survival were "extremely remote".
The single-turbine engine Piper PA-46 Malibu left Nantes for Cardiff at 7.15pm on Monday, and after requesting to descend, lost contact with Jersey air traffic control.
Cardiff signed 28-year-old Sala for a club-record £15 million to bolster their attack and he had been due to start training on Tuesday.
Nantes, for whom Sala played for three and a half years before sealing a move to the Premier League, said on Twitter: "FC Nantes has learnt that the search for the missing plane was called off. This search mustn't stop, @GuernseyPolice
"The club and its supporters strongly ask for the search to find @EmilianoSala1 to go on, for his family and his near relatives. Together for Emi ! #PrayForSala."
A number of Sala's former team-mates also urged rescue workers to continue looking, Nantes captain Valentin Rongier tweeting: "The search can not stop! Help us encourage the police to continue searching. We need everyone. Let's fight as Emi does all the time, please. #PrayforSala @GuernseyPolice."
A number of Sala's compatriots also added their voices to the calls, with Manchester City striker Sergio Aguero tweeting: "We don't want to give up, we want to hold on to hope. That's why I also wish #NoDejenDeBuscar – don't call off the search. From the start, my thoughts have been with Emiliano, his family and friends #PrayForSala."
Another City player, Nicolas Otamendi, said: "I'd like to express my sympathies to Emiliano's family, and urge the police to keep searching for the plane. There's still hope to find them alive. All my support to his family, and let's hope they get an answer urgently."
The Premier League has said there will be a "moment of silent reflection" ahead of games next week.
Nantes coach Vahid Halilhodzic was visibly emotional as he paid tribute to Sala in an interview with reporters at the club's training ground, describing him as a "real warrior".
"We have to respect the decision of the family who, for the moment, have had no confirmation, so hope still exists," he said.
"We are still waiting for a surprise. I hope that arrives very soon.
"It is for that reason that we do not want to hold either a tribute to him or a minute's silence."
Captain Barker acknowledged Sala's family were "not content" with the decision to stop actively searching, but said he was "absolutely confident" no more could have been done and that UK coastguard protocols were followed.
He said he hoped the families of the missing pair found some comfort in the incident remaining open, despite searches ceasing.
"I can't begin to imagine how they must be feeling, but I do understand and I hope they will find some comfort from the fact that we are not closing this incident," Captain Barker said.
"Although we are stopping searching, the incident remains open and we are continuing with our broadcasts to ships and aircraft that are passing through the area and asking them to report anything that may be linked to the missing aircraft."
Mr Barker said the plane was last known to be near Hurd's Deep, a underwater valley north-west of the Channel Islands, which could make recovery of a wreckage very difficult.
Sala's WhatsApp recordings sent from the aircraft may be used in investigation into the events and the weather may have player a role in the crash, Mr Barker added.
Cardiff said in a statement on Thursday evening: "At this time of immeasurable sadness, our thoughts are with the family and friends of both men on board, the fans of both FC Nantes, Cardiff City, and their connections around the world."
The club's owner Vincent Tan added: "Monday evening's news shook everyone at Cardiff City FC to the core.
"We were looking forward to providing Emiliano with the next step in his life and career. Those who met Emiliano described a good-natured and humble young man who was eager to impress in the Premier League.
"The response from the football community has been truly touching and we place on record our sincere thanks to those who have sent messages of support. We also thank everyone involved with the search and rescue operation, and continue to pray for Emiliano, David Ibbotson and their families."
Rescue teams have scanned around 1,700 square miles and examined mobile phone data and satellite imagery but found no trace of the aircraft.
Three planes and five helicopters racked up 80 hours combined flying time looking for the plane, working alongside two lifeboats and other passing ships.