"Britons and people from around the world have joined the international storm of protest against this atrocity in Egypt, with many saying they'll no longer consider Egypt as a possible holiday destination," the group's chief executive, Philip Lymbery, said.
"Egypt's government needs to act quickly to stop this appallingly cruel and totally unnecessary cull and show the world that lessons have been learned. Otherwise, the country's tourist industry could be amongst the casualties."
In 2007, more than one million Briton visited Egypt, more than from any other country, according to tourism ministry and embassy figures.
Last month, Egypt ordered the nation's pigs slaughtered in what it said was a swine flu prevention measure, although the World Health Organisation said the drastic step was unjustified.
In May, another animal welfare group - People for the Ethical Treatment of Animals (PETA) - expressed its "shock and outrage" at the mass slaughter after videos were posted online.
One clip posted by independent newspaper Al-Masri Al-Yom showed gory images of pigs being beaten with iron bars, piglets being stabbed with kitchen knives and animals being kicked alive into bulldozer buckets.