About 18 months ago a new nonprofit group called Free and Fair Markets Initiative launched a national campaign criticizing the business practices of one powerful company: Amazon.com Inc.

Free and Fair Markets accused Amazon of stifling competition and innovation, inhibiting consumer choice, gorging on government subsidies, endangering its warehouse workers and exposing consumer data to privacy breaches. It claimed to have grass-roots support from average citizens across the U.S, citing a labor union, a Boston management professor and a California businessman.

What the group did not say is that it received backing from some of Amazon’s chief corporate rivals. They include shopping mall owner Simon Property Group Inc., retailer Walmart Inc. and software giant Oracle Corp. , according to people involved with and briefed on the project. Simon Property is fighting to keep shoppers who now prefer to buy what they need on Amazon; Walmart is competing with Amazon over retail sales; and Oracle is battling Amazon over a $10 billion Pentagon cloud-computing contract.

The grass-roots support cited by the group was also not what it appeared to be. The labor union says it was listed as a member of the group without permission and says a document purporting to show that it gave permission has a forged signature. The Boston professor says the group, with his permission, ghost-wrote an op-ed for him about Amazon but that he didn’t know he would be named as a member. The California businessman was dead for months before his name was removed from the group’s website this year.

Free and Fair Markets, or FFMI, declined to reveal its funders or disclose if it has directors or a chief executive.