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American 800m runner Nick Symmonds says he has been left out of his country's squad for the World Championships because of a row over what training kit he can wear at team functions.

Symmonds, sponsored by Brooks Running, has refused to sign a mandatory USA Track and Field (USATF) contract that instructs athletes to wear Nike gear.

Symmonds won world 800m silver in 2013.

"I just received word from USATF that I have been left off the Beijing team," the 31-year-old tweeted on Sunday.

Symmonds has hinted he may take legal action over the issue.

"You just can't give a monopoly to a company and expect there to be a healthy, viable sport," he said. "It's never going to change unless someone makes a stand. It's unfortunate that's going to have to be me.

"It's unfortunate it might leave me off the team to Beijing. But it's time to wrestle some of our power back from USATF, because they've taken a lot of it.

"If I'm not on the team, I'm going to sit down and have a chat with my lawyer."

In a statement, USATF said the organisation "respect(s) Nick and whatever decision he makes", but added: "It is the industry standard across all sports that there are certain requirements for when a team uniform and apparel must be worn.

"We all make our best efforts to balance athletes' personal interests with those of the sport, federation and team. Often those interests are aligned, and sometimes they conflict."

The US will officially announce their squad for the World Championships on Monday. The event in Beijing starts on 22 August.