Researchers at the University of Plymouth analysed bricks that had washed up on the coastlines of southwest England.
They confirmed the ages of individual pieces and weighed them, then compared the result with that of equivalent unused pieces.
The study, published in the journal Environmental Pollution, estimated that the bricks could endure for between 100 and 1,300 years.
Dr Andrew Turner, associate professor in environmental sciences, said: “Lego is one of the most popular children's toys in history and part of its appeal has always been its durability.
“It is specifically designed to be played with and handled, so it may not be especially surprising that despite potentially being in the sea for decades, it isn't significantly worn down.
“However, the full extent of its durability was even a surprise to us.
“The pieces we tested had smoothed and discoloured, with some of the structures having fractured and fragmented, suggesting that as well as pieces remaining intact, they might also break down into microplastics.
“It once again emphasises the importance of people disposing of used items properly to ensure they do not pose potential problems for the environment.”
Over the past decade, voluntary organisations from Cornwall - include Rame Peninsula Beach Care and the Lego Lost at Sea Project - have retrieved thousands of pieces of Lego and other plastic during regular beach cleans.
The study used 50 pieces of weathered Lego, constructed of acrylonitrile butadiene styrene, that were washed and then weighed in laboratories at the University of Plymouth.
An X-ray fluorescence spectrometer was used to determine the chemical characteristics of each block, with results used to confirm the age of the pieces based on the presence of certain elements no longer in use.
Researchers then paired the bricks with unweathered sets purchased in the 1970s and 80s.
They were able to identify levels of wear in the bricks retrieved from beaches, which was used to calculate how long the pieces might continue to endure in the marine environment.
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