Chinese officials are planning to ban online brokerages such as Futu Holdings Ltd and UP Fintech Holding Ltd from offering offshore trading services to mainland clients, the latest development in a broad regulatory crackdown that has roiled a wide range of sectors over the past year. The Nasdaq-listed Chinese firms are two of the biggest players in the sector and a ban would block millions of retail investors in mainland China from trading securities easily in markets such as the United States and Hong Kong. Conce­rns over data security and capital outflows are driving the potential ban, sources said. The…