Black Sabbath star Tony Iommi has joined a campaign to urge Indonesian officials to spare the lives of two death row prisoners.

Andrew Chan and Myuran Sukumaran face death by firing squad after being found guilty of drug charges, and the heavy rocker has joined activists appealing to Indonesian President Joko Widodo to show mercy and cancel the executions.

According to the Sydney Morning Herald, Iommi has written a letter to Widodo, in which he suggests Chan and Sukumaran have become "reformed men making a positive difference to the lives of their fellow prisoners."

He writes, "That they have been transformed so much is a real credit to the Indonesian authorities. For this reason, I would ask that you stop the execution of Andrew and Myuran. Please allow them to serve out life sentences where they contribute to the wellbeing of Indonesia and make good for the error of their previous ways. I do hope you will consider this personal appeal."

Iommi reportedly became involved with the prisoners' plights through his connections with Australian charity DoSomething.