Investigations at 16 coastal locations around the estuary uncovered a massive 20,281 pieces of plastic in an area covering just 48 square metres.
On average, 1,268 plastic fragments from various sources were found at each site.
The study was carried out by environmental scientist Suzanne Grimes, from the University of Dundee.
She says plastic pollution in the local environment has reached “overwhelming and disturbing” levels.
Most worrying, she says, is that nearly three quarters of the debris was lentil-sized pellets known as nurdles, which are used in manufacturing but can be deadly to wildlife.
She believes the findings suggest there is mishandling by plastics makers based around the Forth, which is resulting in large quantities of nurdles escaping into the estuary.
The single most polluted place she surveyed was Bo’ness, where 9,671 nurdles and 4,894 items such as cotton buds, cigarette butts and straws were found in an area of beach measuring just three square metres and 2cm deep.
“I was absolutely shocked by what I found when I started this project,” said Ms Grimes, who graduated last year.
“I didn’t expect to find so many heavily polluted areas. It was overwhelming and disturbing. What I found was worse than any previous attempts to gauge the scale of the plastic pollution problem had shown.”
She believes the results show how bad the situation is across the whole Forth estuary.
“I expected to find a lot of plastic bags, bottles, cotton buds and things like that because there’s been so much publicity about them, but the nurdles really shocked me,” she said.
“It’s impossible to say how bad the situation is, but you must be talking about millions of pieces in the Firth of Forth.
“I live in Fife and used to take my children to beaches here but I wouldn’t do that now, having found what I have.”
She is calling for urgent action to tackle the crisis, including placing more pressure on businesses causing the pollution.
“Something needs to be done about this,” she said.
“Millions of pieces of primary plastic are finding their way onto our beaches and into our water and no one is taking responsibility.”
Dr Alison Reeves, a lecturer at the university who has been studying plastic pollution for many years, was “shocked” at the latest findings and warned that cleaning up this sort of “diffuse pollution” is particularly challenging.
She said: “Removing nurdles is not like getting rid of metals. You can’t use a magnet.
“Currently there is no alternative but to sieve and physically pick them up. Once they get into the environment they are hard to remove.
“Larger pieces of plastic are relatively easy to remove but these tiny pieces are being eaten by birds and marine life and it lodges in their digestive systems. They do not have the enzymes in their intestines to reduce the size of the pellets and birds and marine mammals starve to death as their guts remain full of indigestible material.