Aviation authorities were right to ground commercial jets for seven days after last year's volcanic eruption in Iceland, a study has found.
Safety fears led to the action as fine-grained ash from the Eyjafjallajokull volcano blew into the path of aircraft over the UK and continental Europe.
The decision disrupted travel for 10 million passengers and cost between £1.3 billion and £2.2 billion, leading some critics to question whether it was justified.
A new scientific report published today in the journal Proceedings of the National Academy of Sciences concludes that it was.
Researchers analysed samples of ash from the volcano and found they were capable of causing an air disaster. The fragments remained "sharp and abrasive" even after attempts to blunt the particles by stirring them in water.
They would have sandblasted aircraft windows, making them impossible to see through, and had the potential to stall engines.
The explosive nature of the eruption on April 14 last year was caused by glacial meltwater coming into contact with hot volcanic magma. Tiny pieces of extremely hard, abrasive material were ejected to heights of more than nine kilometres and carried as far as Russia.
The researchers, led by Dr Sigurdur Gislason from the University of Iceland, wrote: "In the lab, ash particles did not become less sharp during two weeks of stirring in water, so airborne particles would remain sharp even after days of interaction with each other and water in clouds. Thus, concerns for air transport were well grounded."
Results of the study could form the basis of a safety protocol for rapidly assessing the risk from future volcanic eruptions, said the scientists.