The four-and-a-half inch long string was made from tough stems of honeysuckle, nettles or wild clematis twisted together.
Marine archaeologists discovered it when they found a flooded Stone Age settlement just off the coast of the Isle of Wight.
The team, led by Gary Momber of the Hampshire and Wight Trust for Maritime Archaeology, cut small blocks of the sea floor out for analysis after seeing the wooded remains of the settlement by chance.
The string was buried in one of them. The find is remarkable because the fibres, made of organic matter, would usually decay quite quickly.
Now the results have been published in British Archaeology magazine whose editor Mike Pitts described it as a "fantastic find".
He said: "I don't think the average person realises what an important piece of technology string has been over the ages." (British Archaeology)