Predictions suggest a build-up of about 80,000 tonnes of plastic in the "Great Pacific Garbage Patch" between California and Hawaii.
This figure is up to sixteen times higher than previously reported, say international researchers.
One trawl in the centre of the patch had the highest concentration of plastic ever recorded.
"Plastic concentration is increasing - I think the situation is getting worse," said Laurent Lebreton of The Ocean Cleanup Foundation in Delft, Netherlands, which led the study.
"This really highlights the urgency to take action in stopping the in-flow of plastic into the ocean and also taking measures to clean up the existing mess."
Waste accumulates in five ocean areas, the largest being the patch located between Hawaii and California.