China’s central bank issued an unusual warning to retailers, telling them it is illegal to refuse remittances made with physical cash in favour of mobile and electronic payments.

The People’s Bank of China spoke out after some retailers in major cities began refusing to accept actual cash, while some cities have begun promoting themselves as cashless, South China Morning Post (SCMP) reported.

In a post on social media site WeChat, the authority said refusing to accept cash could exclude people unfamiliar with mobile and electronic alternatives, along with those unable to access these services due to living outside major urban areas.

The bank is also concerned growing acceptance of mobile and electronic payments could ultimately diminish confidence in physical currency.

Its comments come a matter of weeks before a deadline for mobile payment providers including WeChat Pay and Alipay (the country’s two dominant players) to implement new rules covering deposits of prepaid funds with the bank.

These rules were announced in June, at which time Nikkei Asian Review estimated such companies held a combined CNY500 billion ($72.3 billion) in customer funds.