ANDERSON, Ind. -- The NFL won't allow Colts defensive end Robert Mathis to use his face mask on game days.

The NFL won't allow Colts defensive end Robert Mathis to use this face mask on game days. AP Photo/Darron Cummings

League spokesman Brian McCarthy wrote in an email to The Associated Press that the NFL determined it was a prohibited non-standard face mask.

At training camp, Mathis has been wearing cagey-looking white face mask that covers up just about everything except his eyes. Several players around the league including Arizona's Darnell Dockett and the Giants' Justin Tuck have been using similar face masks, and those also will be disallowed. The only exception to wearing a customized face mask, McCarthy said, would be for a medical reason.

Mathis used his Twitter account to explain the decision.

"its official.... the league says i cant wear my facemask. I will return to my run of the mill grill," he wrote.

Mathis is a five-time Pro Bowl player and is second on the Colts' career sacks list with 91½.

He wasn't surprised by the decision.

On Thursday, he used Twitter to explain that he "may not" win the battle with the league office and asked fans: "anybody got pics/suggestions of a face mask within the horizontal4 and vertical3 guidelines?"

One fan even responded by urging Mathis to file an appeal.

Mathis hasn't said what face mask he will use, but presumably it will fall within the bounds of league rules.