Daryl Foster, who coaches Kent, wrote to the Board of Control for Cricket in Sri Lanka saying a video of the bowler from six different angles showed that the apparent throwing was a "visual illusion".
"With respect to Muralitharan's leg spin, all views indicated that he does not extend his arm, therefore his leg-spinning action is also legitimate," Foster said.
The verdict adds weight to Sri Lanka's plea of innocence during its rocky tour of Australia, during which the team was also found guilty and then exonerated of ball tampering.
"It's another opinion from a specialist," Hilary Marcelline, the cricket board's honorary treasurer, said. "It's a split- second thing. What may appear to the naked eye may not do so on video," he said.
Muralitharan was no-balled for throwing during the second Test in Melbourne last month and the International Cricket Council advised him to change his style to ensure his career can continue.Reuse content