According to reports, Spencer, who founded broking firm ICAP in 1986, sold 10.3m shares on January 8 at 440p a share, netting him £45m.
Last week, the company issued a warning to shareholders that profits were set to fall, causing ICAP shares to fall to 297p. Had Spencer sold after the warning he would have realised just £30m.
He had defended his actions, saying “strict procedures” on share sales were followed.
The FSA has refused to comment on whether Spencer will be investigated.