The beloved lion, famed for his black mane, was shot and killed by Minnesota dentist Dr. Walter Palmer during an expedition in Zimbabwe earlier this month (Jul15).

The slaughter, which reportedly occurred after Palmer's guides allegedly lured Cecil out of the protected Hwange National Park, left animal lovers across the world outraged, and the tragedy has prompted Cuoco-Sweeting to step up her activism.

On behalf of The Humane Society of the United States, The Big Bang Theory star has called on U.S. Fish and Wildlife Service head Daniel Ashe to push forward with the immediate addition of African lions to list of animals protected under the Endangered Species Act.

The law would effectively ban the trade of lions and lion parts across the U.S., unless permitted under special circumstances.

In the letter, obtained by WENN, Cuoco-Sweeting adds, "Poachers and hunters from the U.S. and other countries are driving lions to extinction. Fewer than 40,000 African lions - and possibly as few as 23,000 - are estimated to remain today. Lions exist in only one-quarter of their former range and are suffering from loss of habitat and prey. Given the dire situation, the loss of 500 lions to American trophy hunters every year is an inexcusable travesty. The U.S. must play its part in eliminating the threats to these beautiful animals.

"I urge the agency to take swift action and finalize the proposed rule to list the African lion as threatened under the Endangered Species Act to provide the necessary protections to African lions before they disappear from the wild. We must do everything we can to save this iconic species from extinction."