Howell has had a scan on the base of his spine

The 31-year-old Englishman has been plagued by back problems for some time and said he was "gutted" at not being fit enough to defend his title.

"I felt I couldn't risk it," he said. "I hit 50 shots today (Monday) just as I did two days ago to see how it felt. Unfortunately I wasn't pain-free.

Seven players from the world's top 15 will be in the Wentworth field.

"I have had scans on the SI joint (sacroiliac joint at the base of the spine) and it shows signs of strain," said Howell.

"I wanted to give it a shot and I checked with the physio but we felt it was really too soon to play.

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"At the end of the day it was a common sense decision. The scans had shown inflammation and I didn't want to endanger my health for other big events later in the season. It was close, but not close enough."

Howell won by five strokes last year after finishing on a score of 17 under par.