(Bloomberg) — China’s largest cross-border online brokers plummeted in U.S. pre-market trading after a central bank official questioned the legitimacy of their operations amid Beijing’s continuing crackdown on private enterprise.

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Cross-border internet brokers are engaged in “illegal financial activities” because they have no “driving licenses” to operate in China, something that’s unrelated to the convertibility of China’s capital account, Sun Tianqi, a senior PBOC official wrote in an article published on the website of Finance 40 Forum.…