Six domestic express delivery companies are being investigated for alleged collusion after they raised their rates simultaneously in Chongqing.

The Chongqing price bureau said in a statement that the six companies held several meetings earlier this year to discuss an increase and decided to raise their rates simultaneously on Aug 1.

Delivery companies involved in this case include the Chongqing branches of YTO Express Co, ZTO Express Co, Yunda Express Co and TTKD Express Co. Because of the size of these companies, their action amounts to a pact to control market rates and violates China's price law, according to the statement.