NEW YORK -- A member of ExxonMobil’s External Citizenship Advisory Panel wrote a scathing letter this week declaring her resignation from the panel advising the company that until recently was headed by newly sworn-in Secretary of State Rex Tillerson.

In the letter, which was first reported by Motherboard on Thursday, Sarah Labowitz said she is “particularly concerned about the company’s targeted attack on respected civil society organizations through the courts.”

Just last week, ExxonMobil filed another brief in a federal Texas court “advancing an argument that everyday aspects of civil society advocacy with public officials should be treated as an illegal conspiracy,” said Labowitz, a corporate social responsibility expert and co-director of the Center for Business and Human Rights at the NYU Stern School of Business.

Sarah Labowitz, co-director of the Center for Business and Human Rights at the NYU Stern School of Business. Courtesy Sarah Labowitz/©NYU Photo Bureau: Matthieu Asselin

“The brief argues that having a private meeting, conducting a workshop, publishing a report, or advocating that a public official take action are all elements of what you deem an illegal ‘conspiracy,’” she said in the Monday letter, addressed to Ben Soraci, the president of ExxonMobil Foundation and general manager of Public and Government Affairs at ExxonMobil.

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“Many companies face criticism and critique, but few respond with the kind of vehemence and aggressive attack strategy that Exxon has executed over the last year,” Labowitz said. “This approach is especially disappointing because there are much more effective and constructive ways to respond to such criticism.”

William Holbrook, a corporate media relations senior advisor at Exxon Mobil Corporation, told CBS News Thursday: “We regret that she has resigned but respectfully disagree with some of the comments made in her letter.”

“We have never characterized any action by civil society representatives as illegal,” Holbrook said. “What we have done is defend the company, on behalf of all shareholders, from politically motivated investigations that are biased, in bad faith and without legal merit. We did not start this, but will vigorously defend ourselves against false allegations and mischaracterizations of our climate research and investor communications.”

Labowitz says she joined ExxonMobil’s External Citizenship Advisory Panel in 2014. The panel, typically comprised of five members, consists of academics, representatives of non-governmental organizations and former government employees. They are experts in environmental and social topics, according to the company.

Labowitz explained to CBS News why she decided to resign now.

“I think anyone who engages in something like this in an advisory capacity -- there’s the question, ‘Are you most effective inside or outside?’ And I had made a strong argument inside, and then made a judgement based on the most recent legal filing that I would be more effective outside,” she said.

Her resignation came just days after former ExxonMobil CEO Rex Tillerson was sworn in as the new secretary of state.

“I hope that Secretary Tillerson will vigorously defend civil society and he should be asked questions about his views on civil society as he assumes this new role,” Labowitz said.