Martin Luther King must have imagined that the man with the camera so often at his side was doing no more than recording history. But it has been revealed that Ernest Withers – who was on hand to capture King riding newly desegregated buses and the shock of the civil rights leader's allies immediately after his murder – was also an FBI informer.

The double life of one of the most celebrated photographers of the civil rights era was exposed by the Commercial Appeal newspaper in Memphis, which reported that Withers passed on photographs to the FBI along with names and background information about activists and details of schedules.

Withers, who was a police officer before becoming a photographer and died three years ago aged 85, documented the civil rights movement from the beginning, covering pivotal moments such as the murder of Emmett Till in 1955, and the Little Rock school integration showdown. He came to be so familiar and so trusted to King and other leaders that he sat in on strategy meetings.

Records released under a freedom of information request show that from at least 1968, and possibly earlier, he spied on not only black civil rights activists but Catholic priests who supported a Memphis-wide strike by sanitation workers, and political candidates, recording car number plates for the FBI.

Withers also helped the bureau to break a militant black group called the Invaders, which had a following in Memphis in the late 60s.

"He was the perfect source for them," the Pulitzer prize-winning historian of the era, David Garrow, told the Commercial Appeal. "He could go everywhere with a perfect, obvious professional purpose."

Withers was identified by the newspaper after the FBI failed to black out his name on a number of documents related to the agency's spying on civil rights activists, released under freedom of information legislation.

The FBI was fixated on King. The then attorney general, Robert Kennedy, authorised it to spy on the civil rights leader, and the FBI director, J Edgar Hoover, went so far as to bug King in bed.

Andrew Young, a former mayor of Atlanta, said activists had been aware that they were closely watched by the FBI, although no one suspected Withers was an informant.

"I always liked him because he was a good photographer. And he was always [around]," he said. "I don't think Dr King would have minded him making a little money on the side."

One of Withers's sons, Rome, who is also a photographer, said he had been unaware that his father worked for the FBI, but said it did not diminish his work documenting the civil rights movement.

"He had been harassed, beaten, shot at. He was a victim," he told the Commercial Appeal. "At that time, when you are the only black on the scene, you're in an intimidating state."