Organisers of the Golden Globe film and TV awards have pledged to increase diversity by making sure at least 13% of its membership is black.
The Hollywood Foreign Press Association has been criticised after a report said none of its 87 members are black.
More than 100 Hollywood PR firms have signed a letter to say they will advise the stars they represent not to work with the HFPA until changes are made.
They accused the organisation of "discriminatory behaviour".
The HFPA said it was "committed to making necessary changes".
Its members are all international journalists based in Los Angeles, who vote for the winners of the glitzy annual awards.
The HFPA said it would add a minimum of 13 black members, increasing the total membership to at least 100.
The threatened boycott could prove costly to the HPFA because its members often make their living writing articles from press conferences and interviews with film and TV stars.
The letter from the PR group, published on Monday, read: "We call on the Hollywood Foreign Press Association to swiftly manifest profound and lasting change to eradicate the longstanding exclusionary ethos and pervasive practice of discriminatory behaviour, unprofessionalism, ethical impropriety and alleged financial corruption endemic to the HFPA.
"To reflect how urgent and necessary we feel this work is, we cannot advocate for our clients to participate in HFPA events or interviews as we await your explicit plans and timeline for transformational change."
Tina Chen, president and CEO of Time's Up, added in a statement: "We are proud to be in solidarity with the voices of over 100 Hollywood PR firms in calling for the Hollywood Foreign Press Association to radically transform the Golden Globes.
"We agree that anything less than transparent, meaningful change will no longer be acceptable. The entire world is watching."
In its response, the HFPA added: "We also acknowledge that we should have done more."
The group's legal advisers have begun to implement changes, "including a comprehensive review of our governance and code of conduct", it continued.
"While we recognise this is a long-term process, we will continue to be transparent, provide updates, and have confidence in our ability to change and restore trust in our organisation and the Golden Globes.
"As we do so, we invite others in the Hollywood community to join us in advancing racial equity in our industry."
How did this start?
Controversy about the make-up of the HFPA has been circling for some time.
Borat Subsequent Moviefilm, the sequel to 2006's hugely successful Borat, was named best musical or comedy film at last month's awards.
"Thank you to the all-white Hollywood Foreign Press," said Sacha Baron Cohen as he accepted the prize.
Is diversity the only issue?
No. The publicists' accusations of "ethical impropriety" and "unprofessionalism" are nothing new.
It has been suggested repeatedly over the years that the HFPA's small and secretive membership pool means the choices of nominees and winners can be swayed by promotions and freebies from production companies.
In the Los Angeles Times expose, it was also reported that HFPA members were flown to Paris by Netflix in support of its comedy series Emily in Paris, which received two nominations at this year's Globes.
Many critics expressed surprise because the show has had lukewarm reviews, whereas the critically acclaimed I May Destroy You was left out of the nominations.
A source close to the HFPA told BBC News last month that it is common for journalists to attend set visits, premieres and press conferences, adding that members attend 15-20 trips per year on average. They said the airfare to the Emily In Paris set was paid for by the HFPA and not the studio.
The suggestion that such trips have any influence on nominations was "absurd", they added, pointing out that set visits to many other TV shows in the last year had not led to nominations.