Facebook and Intel have become the latest big tech firms to announce they are withdrawing from MWC in Barcelona, citing concerns about coronavirus.
Spanish media is reporting that trade body the GSMA, which organises MWC, will meet to decide whether to cancel the event entirely on Friday.
The GSMA declined to comment on "internal meetings".
Other big brands have told the BBC they are reviewing their plans.
Facebook said "evolving public health risks" were behind its decision.
So far the GSMA has said MWC will still go ahead.
However, one contact in the telecoms sector told the BBC that today was likely to be "a decisive day" for other firms contemplating dropping out.
More than 100,000 people usually attend Mobile World Congress every year, and thousands of firms exhibit. Around 6,000 people travel from China, according to GSMA figures.
In its latest statement, it said more than 2,800 exhibitors were still planning to take part.
Intel explained its decision to pull out: "The safety and wellbeing of all our employees and partners is our top priority, and we have withdrawn from this year's Mobile World Congress out of an abundance of caution."
Huawei said it was still planning to go but was reviewing the situation "on a daily basis". The BBC understands it will be relying heavily on its western-Europe-based staff, rather than sending over many executives from Shenzhen.
Chinese smartphone maker Vivo, which was planning to its launch its product range in the UK market at MWC 2020, announced this morning that it would be withdrawing, saying "the health and safety of our employees and the public are our top priority".
TCL, another Chinese firm, said it was cancelling its global press event but would offer "private pre-briefings" and would still exhibit at the trade fair.
Royole, creator of the first foldable phone to come to market, the FlexPai, is another company to drop out. It said it would instead announce its next device - the FlexPai2 - ahead of MWC.
Cyber-security firm McAfee also dropped out on Tuesday, citing health and safety concerns.
Google and Microsoft are still on board.
Nokia said it was currently planning to attend but chief marketing officer Barry French added that "the situation is fluid".
At least one of the big UK phone networks is understood to have significantly reduced its presence at the show.
NTT DoCoMo, Amdocs and Commscope are among those cancelling their plans to go to Barcelona altogether.
The decision to pull out could prove expensive for smaller firms in particular.
Israel/New York-based software startup Lightico told the BBC it expected to lose the $15,000 (£12,000) it had invested in its place at the event by not attending.
It will be hosting what it described as "a virtual booth" instead.
"The main value for us [in attending MWC] was driving business by meeting with prospects, partners, existing customers," said spokesman Eytan Morgenstern,
PR firm Hoffman Agency said it had clients which had dropped out but it was still attending MWC with others - and was looking into ways in which companies that had pulled out could recycle any promotional material and merchandise created for the show.
The GSMA is insisting that all visitors from China will have to be quarantined for two weeks prior to the start of the show.
It is also encouraging delegates to follow World Health Organization hygiene advice, and not to shake hands with each other. It will be carrying out body temperature screening, it said.
Analysis: Cristina Criddle, technology reporter, BBC News
The technology companies might be alarmed by the potential spread of coronavirus at Mobile World Congress this year, but Barcelona itself appears more relaxed about it.
I spoke to several people who live and work in the city and none was overly concerned. One person described the tech firms' reactions as "hysterical".
Barcelona relies heavily on tourism. It has the largest port in the Mediterranean and welcomes millions of people to Catalonia each year from all over the world - more than 100,000 attend Mobile World Congress alone.
It is more fearful about damage to its economy; hotels are already being hit with cancellations, and bars, restaurants and music venues may see a dip in profits this month if many people stay away.
And in a nod to another Barcelona industry that thrives at these times, one anonymous MWC delegate, speaking to the Financial Times, said he "felt sorry for all the pickpockets".