UnitedHealth Group has raised concerns with the top US securities regulator over a social media post by activist investor Bill Ackman, who claimed that the healthcare group could be inflating its profits.

UnitedHealth shares fell more than 4% on Wednesday before finishing down 1%, a day after hedge-fund manager Bill Ackman wrote a post on social media that said the insurer could be overstating its profits because it denies patients medically necessary care.