LOS ANGELES: One of the most celebrated photographers of the US civil rights era, who was part of Martin Luther King jnr's trusted inner circle, has been unmasked as an FBI informant.

Ernest Withers, who died in 2007 at the age of 85, took some of the most famous pictures of King, including him riding one of the first desegregated buses in Montgomery, Alabama.

Trusted ... a photograph of Martin Luther King by Ernest Withers.

Known as the ''original civil rights photographer'', he also captured thousands of images chronicling everyday lives of struggle in the Deep South, along with demonstrations, riots and criminal trials.

One of his most important photographs was taken in 1968 during a Memphis sanitation workers' strike when a crowd stood with placards reading ''I Am a Man''.