Los Angeles - Injury-plagued Indianapolis Colts quarterback Andrew Luck stunned the NFL on Saturday by announcing his retirement from the sport.

Luck, the number one pick in the 2012 draft, said he had opted to call time on his career after struggling to regain full fitness.

"This is not an easy decision. Honestly, it's the hardest decision of my life. But it is the right decision for me," Luck told reporters after the Colts' pre-season game against the Chicago Bears on Saturday.

"For the last four years or so, I've been in this cycle of injury, pain, rehab, injury, pain, rehab, and it's been unceasing, unrelenting, both in-season and offseason, and I felt stuck in it. The only way I see out is to no longer play football."

The 29-year-old has been battling a left leg injury during the pre-season, which has been described variously as a calf and ankle problem.

Luck's decision comes after a mixed record in recent years.

He suffered an injury to his throwing shoulder during the 2016 season which required surgery.

His rehabilitation from the injury forced him to miss the entire 2017 season but he put in a stellar performance when he returned last year.

He finished second in the NFL for touchdown passes and led the Colts into the playoffs for the first time since 2014.

His play saw him win the NFL's Comeback Player of the Year award for 2018, and earned him a fourth Pro Bowl selection.

News of Luck's retirement came as he watched from the sidelines as the Colts played the Bears on Saturday. Furious Colts fans appeared to boo the quarterback as he walked down the sideline.

However Luck said he had no doubt he had made the right decision.

"I haven't been able to live the life I want to live," Luck said.

It has "taken the joy out of this game, and after 2016, when I played in pain and was unable to regularly practice, I made a vow to myself that I would not go down that path again," he said.

"I find myself in a similar situation and the only way forward for me is to remove myself from football and this cycle that I've been in."