Shares on the the Shanghai Composite index in China fell by 5.5% on Friday as some of the county's biggest brokerage firms announced that they were under investigation.

Earlier this week, China's securities association said that Citic Securities had overstated its underlying businesses by $166 billion. And late on Thursday, regulators announced that it would be investigating Citic over its conduct.

Authorities says it will investigate whether or not Citic broke security market rules. Citic's rival Guosen Securities is also being investigated - both firms saw shares fall by 10%, the maximum allowed.

Meanwhile shares in China Haitong Securities were suspended after reports suggested that it was also under investigation.