The NRL has formally issued breach notices to two spectators who allegedly racially abused South Sydney Rabbiohs skipper Greg Inglis at Panthers Stadium.

Inglis was allegedly called a "black dog" by members of the crowd after South Sydney's loss to the Penrith Panthers in round two, with NRL CEO Todd Greenberg vowing to "throw the book" at those responsible if the offensive comments are proven.

An NRL spokesman said the two crowd members have until Thursday to respond to the notices before any penalty is imposed by the governing body.

It's understood one of the spectators is a Panthers member, whose membership has been suspended pending the investigation.

Both the Panthers and Greenberg have publicly apologised to Inglis in the wake of the incident, with the NRL boss last week declaring those responsible "would not be welcome back in the game" if the claims were proven.