Casey ran up a round of 85 on Thursday

Casey, 27, was rank last after a 15-over first-round 85 and offered no reason for his withdrawal, while Howell (74) blamed a torn abdominal muscle.

"Everything was fine and dandy and then I felt a twinge," said Howell. "Nothing untowards and I thought it'd wear off.

"Then I hit a three-iron and I just got what felt like a rip."

Howell, 29, tied for 11th on his Masters debut two months ago.

He missed the cut in his only other US Open appearance, at Bethpage Black in 2002.

It's unbelievably frustrating considering that I might be out for a few weeks right in the middle of the season David Howell

He added: "I've never had this happen in 20 years of golf and there were no signs of it physically this week at all.

"But I hit it in the rough nine times yesterday and hacking out tightened things up a little bit and this morning just put it over the edge.

"It's very annoying. I've had a few injuries over the years, but nothing ever like this.

"It's unbelievably frustrating considering that I might be out for a few weeks right in the middle of the season.

"Hopefully, fingers crossed, it won't be as bad as it feels it might be."

It was Casey's third appearance in the year's second major and he has still to make the cut.

Casey has suffered a slump in form since winning the WGC-World Cup of Golf for England with Luke Donald seven months ago.

He opted to spend the early part of the season on the US Tour but missed the cut in four of six events.

In Europe, he won the TCL Classic in China in March, but has since failed to make the cut in three of five events and is without a top 25 finish.

"I have become paralysed over the ball," Casey told reporters before the start of the US Open.

"I was beating myself up on the course. But all you can try to do is smile and battle through it."

Casey became embroiled in controversy last year when he was quoted as saying he "properly hates" Americans during the Ryder Cup.

Casey claims his quotes were taken out of context.