By Karen Jacobs

ATLANTA (Reuters) - Olympic 400 meters champion LaShawn Merritt, cleared to run at the 2012 Games by a court ruling Thursday having previously been barred for doping, said he would pull out all the stops to succeed in London.

"I'll train hard, harder than I've ever trained," the American told reporters after the Court of Arbitration for Sport (CAS) in Switzerland said he and dozens of past drug offenders were eligible to participate in the London Games.

The court dismissed the International Olympic Committee rule, introduced three years ago, that banned athletes suspended for doping for six months or longer from competing in the next Games.

In 2010 Merritt received a 21-month ban after having tested positive for a banned substance. The suspension ended in July and he competed at the world championships the following month.

"The past couple of years have been rough but I've had a strong support system," said the 25-year-old before adding his family and coach had helped him get through the ordeal.

Merritt endured "many restless nights" but said the whole experience made him grow mentally and spiritually and to become a better person.

Merritt said he was not confident the CAS would rule in his favor and did not want to pick up the phone when his agent, Kimberly Holland, called him early Thursday.

She initially told Merritt the ruling had not gone his way as a joke.

The American said he felt no pressure going to London as the reigning champion and the likely favorite to win gold.

"It's what I love to do," Merritt said. "Just being away from the sport motivated me so much more to be at my best when I return.

"I never quit, I never give up and I'm always going to give it 110 percent."

(Editing by Tony Jimenez)