After an offseason of speculation, the Detroit Lions confirmed on Tuesday that All-Pro receiver Calvin Johnson has retired from the NFL.

Johnson, nicknamed Megatron, filed his retirement papers with the NFL in advance of the new league year, which begins at 4pm Eastern Time on Wednesday. The Lions announced "[contract] matters were settled to the satisfaction of the parties."

Via a statement, Johnson said: "After much prayer, thought and discussion with loved ones, I have made the difficult decision to retire from the Lions and pro football.

"I have played my last game of football. To the fans of Detroit and Michigan. I so appreciated your passionate support over the years and truly enjoyed playing for you. I loved playing in Detroit and will forever be a Lion. My biggest regret is that I wasn't able to help give our fans a championship."

The Lions will gain about $11.1 million in salary-cap space from Johnson's retirement and can target a wideout in free agency or address team needs on the offensive line while drafting a receiver with the 16th overall pick.

Johnson, who turned 30 in September, led the Lions in receiving yards (1,214) and touchdowns (nine) this season despite battling a sprained ankle in the second half of the season. He was listed as questionable on the team's final eight injury reports.

The second overall pick in the 2007 NFL Draft finishes his career with 731 receptions for 11,619 yards and 83 touchdowns - all Lions franchise records.

His decision represents another blow for Lions fans who watched as future Hall of Fame running back Barry Sanders also ended his career early, retiring after the 1998 NFL season at only 31 years old.