(CNN) -- Major League Baseball said Monday that it is suspending New York Yankee Alex Rodriguez for 211 regular-season games through the 2014 season amid allegations involving the use of performance-enhancing drugs.
The 38-year-old slugger said he plans to appeal his suspension.
"I am disappointed with the penalty and intend to appeal and fight this through the process. I am eager to get back on the field and be with my teammates in Chicago tonight," Rodriguez said in a statement. "I want to thank my family, friends and fans who have stood by my side through all this."
His suspension is set to go into effect on Thursday, the league said. But officials also said that Rodriguez could keep playing if he appeals.
Officials also said that 12 Major League Baseball players have accepted 50-game suspensions without pay in connection to an investigation of performance-enhancing drug use.
A-Rod and the other players are accused of having ties to the now-shuttered Biogenesis anti-aging clinic in south Florida and taking performance-enhancing drugs. Rodriguez has denied the accusation.
The Major League Baseball Players Association said Monday that it's standing behind Rodriguez.
"We agree with his decision to fight his suspension," the union said in a written statement, adding that it believed the league's commissioner "has not acted appropriately."
The New York Yankees declined to comment on Rodriguez's suspension until the completion of the appeal process.
"In the meantime," the team said in a written statement, "the Yankees remain focused on playing baseball."
The team did say it was disappointed by the suspension of Yankees catcher Francisco Cervelli, who was one of the 12 players who agreed to the 50-game suspension.
"It's clear that he used bad judgment," the Yankees said.
Before Monday's announcement, the Biogenesis scandal had already ensnared one star: 2011 National League MVP and Milwaukee Brewers outfielder Ryan Braun.
Last month, Braun was suspended without pay for the rest of this season for violating the league's drug policy, baseball Commissioner Bud Selig announced.
On Monday, Selig said the league had no choice but to investigate the allegations in order to "maintain integrity, fairness and a level playing field."
"Despite the challenges this situation has created during a great season on the field," he said, "we pursued this matter because it was not only the right thing to do, but the only thing to do."
'I'm going to play him'
Rodriguez has missed the entire 2013 season so far after undergoing hip surgery, and he told reporters Saturday he plans to rejoin his teammates for Monday night's game in Chicago against the White Sox.
Before the league's decision was announced, Yankees Manager Joe Girardi said he was ready to put Rodriguez in Monday's lineup.
"If he's in there, I'm going to play him," Girardi told reporters Sunday night.
Major League Baseball issued a statement saying that Rodriguez was being disciplined under both baseball's Joint Drug Prevention and Treatment Program and the players' collective bargaining agreement.
Under the drug program, "Rodriguez's suspension will be stayed until the completion of his appeal if Rodriguez files a grievance challenging his discipline," the MLB statement said.
That is also the usual case for suspensions related to drug offenses under the collective bargaining agreement, but the statement did not specifically address that aspect.
Rodriguez is considered one of the game's greatest sluggers. He has 647 home runs -- the fifth most ever -- in 19 seasons.
In 2009, he had an outstanding postseason as he helped the Yankees win their most recent World Series title.
He holds the largest contract ever in American sports, signing with the Yankees in 2007 for $275 million over 10 years.
Rodriguez said Friday he believes his contract makes him an attractive target for a baseball ban or suspension.
"There's more than one party that benefits from me not ever stepping back on the field -- and that's not my teammates and it's not the Yankees fans," he said.
Other players suspended
The other players suspended by Major League Baseball Monday are:
Philadelphia Phillies pitcher Antonio Bastardo
San Diego Padres shortstop Everth Cabrera
New York Yankees catcher Francisco Cervelli
Texas Rangers outfielder Nelson Cruz
Padres pitcher Fautino De Los Santos
Houston Astros pitcher Sergio Escalona
Yankees outfielder Fernando Martinez
Seattle Mariners catcher Jesus Montero
Free agent pitcher Jordan Norberto
Detroit Tigers shortstop Jhonny Peralta
New York Mets outfielder Cesar Puello
Mets infielder/outfielder Jordany Valdespin
De Los Santos, Escalona, Martinez, Montero, Puello and Valdespin are currently on minor league rosters.
In written statements, several teams said they backed the league's decision.
Cruz said he had made an "error in judgment" after an infection left him concerned about whether he would be physically able to play.
"Faced with this situation, I made an error in judgment that I deeply regret, and I accept full responsibility for that error. I should have handled the situation differently, and my illness was no excuse," he said.
"I look forward to regaining the trust and respect of the Rangers organization, my teammates, and the great Rangers' fans, and I am grateful for the opportunity to rejoin the team for the playoffs."
CNN's Adam Reiss reported from New York, CNN's Jason Carroll reported from Chicago and CNN's Catherine E. Shoichet reported from Atlanta. CNN's Steve Almasy and David Close contributed to this report.