Tiger Woods announced on Tuesday he was withdrawing from this week's Arnold Palmer Invitational, one of his favorite events and a key lead-up to next month's Masters.

The world No 1 said he pulled out of the tournament because he was still having problems with his aching back and was unsure whether he would be fit to play in this year's first major.

Woods posted a statement on his website saying he told Palmer in person he would not be able to defend the title he won at Bay Hill last year.

"I personally called Arnold today to tell him that, sadly, I won't be able to play in his tournament this year," Woods said.

"I would like to express my regrets to the Orlando fans, the volunteers, the tournament staff and the sponsors for having to miss the event."

Woods has been plagued by back problems since last year, but his condition worsened over the past month.

The 38-year-old American failed to finish the Honda Classic at Palm Beach Gardens earlier this month, quitting after 13 holes in his final round.

Then he tweaked his back again on the last day at the WGC-Cadillac Championship in Miami a week later, tumbling out of contention with a final-round 78 to finish tied for 25th.

Woods won the last of his 14 majors in 2008 and said he was uncertain whether he would be able to play in the Masters, starting at Augusta National on April 10.

"Unfortunately, my back spasms and the pain haven't subsided," he said.

"It is too early to know about the Masters, and I will continue to be evaluated and work closely with my doctors."

The Arnold Palmer Invitational, starting on Thursday, is one of Woods's favorite events and attracts most of the world's top players.

Woods has won the tournament eight times, including each of the past two years, and would have been chasing an unprecedented ninth title.