The high levels of the pollutant represent a dramatic increase on similar surveys carried out just six years ago.
Research also showed the presence of a species of shrimp not normally found this far north, which could be an indicator of climate change and warming seas.
The data was collected during summer 2022 by teams competing in the GB Row Challenge, a 2,000-mile event that circumnavigates Great Britain.
The aim of the research project was to build a picture of the many challenges facing British coastal waters. Using specialist equipment, the rowers gathered data on microplastics, temperature, noise pollution and biodiversity.
In 2017, Cefas (Centre for Environment, Fisheries & Aquaculture Science) published microplastic data from many of their offshore trawls.
It found just 0-1.5 microplastic particles per cubic metre (m3) that were bigger than 0.3mm in size. The samples collected during the Challenge event show much higher concentrations, with almost 100 times more microplastics in some areas.
Scientists say the main reason for the big difference is the smaller sizes of particles that were captured in special steel filters. Nearly all of the microplastics collected by the GB Row teams were smaller than 0.3mm.
Dr Fay Couceiro, a researcher from Revolution Plastics at the University of Portsmouth, said: "Ocean pollution is one of the biggest challenges of our generation. The data collected by GB Row Challenge will greatly enhance our understanding of conditions in the seas around the UK.
"The equipment used to collect data during the event has enabled us to capture much smaller particles, so we have been able to get a more accurate picture of where and how concentrated microplastics are. Over time it will significantly improve our understanding of the challenging problem of microplastics in our water."
As well as higher concentrations in coastal waters, the initial research data also showed up to four times more microplastics in the Thames than was collected in October 2017.
A previous study found a maximum of 36.7 microplastics per cubic metre in Putney, whereas this study found 121 microplastics in the Thames Estuary. Although this variance in data may be due to the different sampling locations, methods and time of year, it is thought likely that the concentration of microplastics are increasing.
Microplastics are pieces of plastics smaller than 5mm. They may be plastics deliberately made that size or small pieces of plastic that have broken off from larger pieces.
Scientists began to notice microplastics in the oceans almost 20 years ago. Since then, methods for detecting them have improved and studies have been conducted to determine if they are harmful. Most of these studies have taken place in sea animals and unfortunately the results are troubling.
"There is currently no complete map for the UK concentrations of microplastics in our coastal waters," Dr Couceiro said. "These comparisons really show the need for a comprehensive map of these smaller-sized microplastics and an annual monitoring method, which we have begun with this University of Portsmouth and GB Row Challenge collaboration.
"The long-term aim is to collect these datasets for each GB Row Challenge between 2022 and 2025, which will give a great baseline for the entire UK and also show any changes happening over that time."
