Panthers franchise center Ryan Kalil

The Carolina Panthers on Tuesday used the "non-exclusive" franchise tag on center Ryan Kalil.

Kalil is planning on signing the team's franchise offer sheet, a league source told ESPN NFL Insider Adam Schefter. The players' union has questioned the validity of the franchise tag for next season with the current collective bargaining agreement set to expire, so Kalil's decision to sign the offer is an interesting one.

The 25-year-old Kalil is coming off his second straight Pro Bowl season. Under the franchise tag, Kalil is offered a guaranteed one-year deal at about $10.5 million.

Kalil, who was drafted in the second round of the 2007 draft, is the first center to receive the franchise tag since the Lions' Kevin Glover in 1996.

The Panthers' decision will allow running back DeAngelo Williams and defensive end Charles Johnson to become unrestricted free agents and potentially leave Carolina.

"It was a difficult decision," Panthers general manager Marty Hurney said. "There were several guys we were considering but we only had one tag. In the end, we decided to use it to keep our offensive line intact."

Drafted 27th overall in the 2006 draft, the 27-year-old Williams holds the Panthers' franchise mark for career rushing yards (4,211) and rushing touchdowns (31). In 2008, he led the NFL with 18 rushing touchdowns and 20 total scores. He made his only Pro Bowl appearance in 2009.

Johnson, 24, led the Panthers with 11½ sacks. He was drafted in the third round of the 2007 draft.

The Panthers have the option to take a defensive end with the No. 1 overall pick in the draft, possibly Clemson's Da'Quan Bowers. The Panthers also need help at defensive tackle, with Auburn's Nick Fairley seen as a potential candidate for the top pick, too.

After rushing for 1,515 yards in 2008 and 1,117 in 2009, Williams' numbers declined dramatically last season as the Panthers became the NFL's lowest-scoring team.

He managed only 361 yards and was averaging 4.1 yards a carry -- a yard worse than his career average going into the season -- when he injured his foot late in a victory Oct. 24 against San Francisco.

When the Panthers suffered numerous other injuries and needed to sign an offensive lineman, the Panthers placed Williams on injured reserve Nov. 17.

In 2009, Williams and Jonathan Stewart, 23, became the first teammates in NFL history to each rush for more than 1,000 yards. Stewart is signed through the 2012 season.

Information from ESPN NFL Insider Adam Schefter, ESPN.com senior writer John Clayton and The Associated Press was used in this report.