Small investors in shares of in Spain's nationalised lender Bankia suffered new massive losses on Thursday as the stock plunged by more than 50 percent amid an abnormally high volume of trading which the stock market regulator said would be looked at closely.

Tens of thousands of small savers, who were often missold preference shares and hybrid debt in Bankia, swapped their investment for ordinary shares on Thursday at a an average discount of around 40 percent.