The announcement followed a crisis meeting of the GSMA board, after its hand was forced by the pullout of anchor European members including Deutsche Telekom, Vodafone, BT and Nokia.
Spain's Telefonica, one of the biggest telecom operators not to have announced it was pulling out, said on Wednesday evening it "understands the GSMA's decision to cancel the Mobile World Congress 2020 due to the situation generated by coronavirus."
It said it would always support Barcelona as host city of the Mobile World Congress.
Barcelona's mayor, Ada Colau, said earlier she wanted to send a "message of calm," insisting the city was ready to host the event, while Spanish health officials reiterated that there was no reason to call off MWC.
The World Health Organisation, a UN agency leading the coronavirus crisis response, had also called in vain for calm.
“There is no evidence at present to suggest that there is community spread outside China, so WHO is not currently requesting that large gatherings are cancelled," WHO spokesman Tarik Jasarevic told Reuters in Geneva.
That failed, however, to alleviate concerns among major exhibitors that the precautions would be insufficient to halt the virus, which has spread beyond China's borders to two dozen countries.
"To bring people together and connect them: That is what Telekom stands for. This is also what the Mobile World Congress, the 'class reunion' of our industry, stands for," Deutsche Telekom CEO Tim Hoettges posted on LinkedIn.
But he said that large gatherings of people with many international guests posed a particular risk: "To take this risk would be irresponsible."
The Chinese contingent at MWC has numbered 5,000-6,000 in recent years, making the event particularly vulnerable given the outbreak of the virus that has killed more than 1,100 people on the Chinese mainland.
The GSMA said the host cities and partners respected and understood its decision, adding that they would "continue to be working in unison" towards staging next year's event.
Hoffman and Colau are to hold a joint press conference on Thursday.
Due to the sheer scale of MWC, with delegates packing out Barcelona's hotels and restaurants and causing the city's trade fair grounds to burst at the seams, postponement was never a realistic option, sources said.
ON THE HOOK
A final decision was made harder by the terms under which any events insurance taken out by the GSMA would pay out, industry sources and insurance experts said. This would be unlikely to kick in unless restrictions are imposed on public gatherings in the country on health grounds.
"Where there is no ban and businesses make their own commercial decision, I cannot see the market paying out," Edel Ryan, who is on the Special Risks team at broker Marsh JLT Specialty, said before the GSMA's decision.
Major Chinese exhibitors, led by Huawei HWT.UL , had stuck to plans to attend to the very last, ordering at-risk staff to isolate themselves in advance and drafting in replacements from elsewhere to run event stands and host clients.
The GSMA had banned attendees from China's Hubei province, where the coronavirus outbreak began, and required others to prove that they had been outside the country for at least two weeks prior to the event.
Coronavirus has proved to be contagious even when people who have caught it are asymptomatic, meaning that people attending might not even realise that they could infect others at MWC. Tracking the meetings and movements across the Fira trade grounds and the city of Barcelona of anyone who later tests positive would be a difficult task.
(Writing by Douglas Busvine, Additional reporting by Jessica Jones, Isla Binnie, Supantha Mukherjee, Joan Faus, Can Sezer, Tarmo Virki, Stephanie Nebehay, Carolyn Cohn and Noor Zainab Hussein; editing by Chizu Nomiyama, Kirsten Donovan and Leslie Adler) ((email@example.com; +49 30 2888 4084; Reuters Messaging: firstname.lastname@example.org))