Two scientists resigned from a key Environmental Protection Agency science advisory board on Friday in an act of protest against a decision this week removing the board's co-chairmen, according to a resignation letter sent by one of the scientists.

"It is with certain regret and concern — and in protest — that we submit our resignations as members of the Sustainable and Healthy Communities Subcommittee of the Board of Scientific Counselors" for the EPA, says the letter from scientists Peter Meyer and Carlos Martin.

Martin tweeted the letter, saying, "Just resigned from EPA subcommittee to protest removal of [Robert Richardson] @ecotrope & Courtney Flint," who headed the board. "Painful professional decision," Martin added.

The Union of Concerned Scientists, an advocacy group representing environmental scientists, said the resignations were "largely due to the administration's efforts to water down credible science and the recent removal of the subcommittee's co-chairs," according to spokesman Seth Michaels.

EPA officials have been adamant that the departure of the co-chairmen and several others earlier this week was not a dismissal or firing. The agency simply did not seek to renew a number of scientists' terms on the board and allowed them to expire, the EPA said.

The board is comprised of outside advisers from business and academia who make scientific recommendations on proposed EPA research and regulations.

"Advisory panels like [the Board of Scientific Counselors] play a critical role reviewing the agency's work," EPA spokesman J.P. Freire told the Washington Examiner in an email. "EPA received hundreds of nominations to serve on the board, and we want to ensure fair consideration of all the nominees – including those nominated who may have previously served on the panel – and carry out a competitive nomination process."