Critically-endangered vaquitas could become extinct in a matter of months, with fewer than 30 of the porpoises left, conservationists have warned.
Urgent action to clamp down on illegal fishing is needed to save the "panda of the sea", wildlife charity WWF said.
Vaquitas, the world's most-threatened marine mammal, are only found in Mexico's Upper Gulf of California, where their population has declined by 90% since 2011 to fewer than 30 individuals in the latest estimates.
The porpoises are being hit by unsustainable fishing practices and illegal trafficking of the critically-endangered totoaba, a fish whose swim bladder is prized in China as a delicacy and for medicine.
Vaquitas get caught in the gillnets - fishing nets which are hung vertically to trap fish - set to catch species such as the totoaba.
A report into the plight of the world's smallest porpoise from WWF comes as the current two-year ban on gillnets is set to expire.
WWF warned that despite some efforts by the Mexican government to enforce the ban, illegal gillnet fishing has continued.
The charity called for a permanent ban to be introduced and enforced for all gillnets.
Action is also needed to remove "ghost nets", fishing gear that has been lost or abandoned in the water, to prevent the vaquitas being accidentally caught and drowned.
WWF is also urging the US and Chinese governments to work with Mexico to intercept and halt the illegal transport and sale of totoaba products, which it says are smuggled through the US to China.
Chris Gee, head of campaigns at WWF-UK, said: "Time is rapidly running out for the vaquita, we could tragically lose the 'panda of the sea' in a matter of months.
"We need the public's help now to motivate the Mexican government to act to protect the species and the World Heritage site that provides home to the vaquita.
"The last hope for the species is the Mexican government immediately putting in place and properly enforcing a permanent ban on gillnets.
"This will also help safeguard this precious World Heritage site and the livelihoods of the local people who depend on it."
The Islands and Protected Areas of the Gulf of California World Heritage site is home to more than a third of the world's marine mammal species, five of the world's seven turtles and almost 900 fish species as well as many sea birds.