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Opting out does not mean that James has decided to leave the Heat, sources said.
James had until June 30 to decide whether to opt out of the final two years of his contract with Miami. He was scheduled to make $20 million next season and had two years and about $42.7 million remaining on his deal.
"I was informed this morning of his intentions. We fully expected LeBron to opt out and exercise his free-agent rights, so this does not come as a surprise," Heat president Pat Riley said in a statement. "As I said at the press conference last week, players have a right to free agency and, when they have these opportunities, the right to explore their options.
"The last four seasons have been historic, and LeBron James, Dwyane Wade, Chris Bosh and Erik Spoelstra have led the Miami Heat to one of the most unprecedented runs in the history of the NBA. We look forward to sitting down with LeBron and his representatives and talking about our future together. At the moment, we are preparing for the opportunities in the draft and free agency as we continue with our goal of winning NBA championships."
After Miami's season-ending loss to the San Antonio Spurs in Game 5 of the NBA Finals, James told reporters he would take a vacation with his family before considering his contract options.
James indicated that he welcomed the opportunity to become a free agent and have the same level of flexibility he was afforded in 2010, when he signed with Miami after spending his first seven years in Cleveland.
"Being able to have flexibility as a professional, anyone, that's what we all would like," James said last week. "That's in any sport, for a football player, a baseball player, a basketball player, to have flexibility and be able to control your future or your present. I have a position to be able to do that. ... There's a lot of times that you're not in control of your future as a professional."
In his four seasons in Miami, the Heat have gone to the NBA Finals four times, winning two championships. James could join Shaquille O'Neal as the only players in league history to lead the NBA Finals in total points scored and then play for a different team to start the following season, according to the Elias Sports Bureau. O'Neal played for the 2003-04 Los Angeles Lakers, who lost to the Detroit Pistons in the NBA Finals, and was traded to the Heat in the offseason.
If James departs, he would be leaving a historic opportunity on the table. Only one team in NBA history has appeared in at least five consecutive NBA Finals, the Boston Celtics, who reached 10 straight from 1957 to 1966.
With James and Carmelo Anthony officially opting out of their contracts, multiple teams -- including the Cleveland Cavaliers, Atlanta Hawks and Lakers -- have begun the process of exploring roster moves that would create sufficient salary-cap space to sign 2014's marquee free agents in tandem, sources close to the situation told ESPN.com.
Meanwhile, the other two Miami stars with early termination options -- Wade and Bosh -- have not yet told the Heat or publicly revealed whether they will join James as free agents this summer.
"No news here yet," Wade said Tuesday.
Bosh and James both said last week that the three would meet before deciding anything about their respective futures, and it has been widely speculated that they might have to rework their deals to keep the Big Three together in Miami.
Each signed six-year deals when they famously teamed up in Miami in 2010, and those deals came with options to become free agents either this summer or in the summer of 2015. Together, they have won four Eastern Conference titles and two NBA championships, winning more games than any other team in the league over that span.
"There's a conversation that will be had between the three of us," James said last week. "I think it's only right. I think we've earned that for each other, to have a conversation and see what could possibly happen."
News of James opting out Tuesday also affected Las Vegas, where several sports books, including the Wynn, took the drastic measure of pulling all future odds on all NBA teams to win next year's title.
Information from The Associated Press and ESPN Stats & Info was used in this report.