China's central bank has drafted rules for a tax on foreign-exchange transactions that would help curb currency speculation, according to people with knowledge of the matter.

The initial rate of the so-called 'Tobin tax' may be kept at zero to allow authorities time to refine the rules, said the people, who asked not to be identified as the discussions are private.

China has been fighting the impact of speculators on its currency. Credit:iStock

The tax is not designed to disrupt hedging and other foreign-exchange transactions undertaken by companies, they said.

Imposing a levy on foreign-exchange trading would be the most extreme step yet by policy makers to prevent speculative bets against the Chinese currency, after state-run banks repeatedly intervened to support the yuan and the government intensified a crackdown on capital outflows.