LONDON—U.S. aluminum industry figures urged their European colleagues to join them in acting against what they called China’s illegal subsidizing of its aluminum industry, after publishing an open letter to U.K. Prime Minister Theresa May on the matter on Wednesday.

The China Trade Taskforce, a campaigning body made up of industry figures and trade lawyers, claims that China had unfairly subsidized its heavy industries and breached World Trade Organization rules in doing so.

Representing the aluminum industry, the group called on Prime Minister Theresa May to “actively engage with the WTO on this matter and press for action.”

The prime minister’s office had no immediate comment.

The action comes after the Trump administration announced recently it would open an investigation to ascertain the extent to which cheap Chinese aluminum imports “impair the national security.”

While the Obama administration in December launched a formal complaint with the WTO about Chinese aluminum subsidies, last week’s announcement by the U.S. marks a shift of focus to aluminum, after European nations and the U.S. last year enacted measures to protect their steel industries from Chinese dumping.

When China joined the WTO in 2001, its domestic aluminum demand justified its industry’s growth rate, according to the CEO of Century Aluminum, Michael Bless. However, with the forecast for China’s aluminum demand growth at 5% to 6% this year but its supply seen expanding 11% to 12%, the Chinese industry “has no other advantage than state support,” he said Wednesday.

While China now accounts for 56% of global production, the European Union’s share of the global market has fallen to 7%. “If nothing changes, the European aluminum market will soon cease to exist,” the task force said.

The campaign’s open letter to the prime minister comes as the world’s largest aluminum producer, China Hongqiao Group Ltd., faces fraud allegations for underreporting production costs and failing to disclose electricity expenses.