Mike Thomas, the Washington Redskins' leading ground gainer the last three seasons, has decided to play out his option and become a free agent after the 1978 season, it was learned last night.

Redskin General Manager Bobby Beathard confirmed that negotiations with Thomas and his attorney, John Dickens of Dallas, were at an impasse and that "Mike and his agent feel it would be best for him to play out his option.

It's their decision. We couldn't agree about anything. No, we don't plan to re-open negotiations. If they want to, fine. We finished talking about two days ago, and this was the decision they've made."

Thomas was not available for comment, but Dickens said, "The offer they made us was one that we just could not accept. There was a hang-up on the money and the guarantee. No further negotiations are planned."

In other Redskin developments yesterday, quarterback Billy Kilmer passed his team physical and said he planned to be in Carlisle, Pa., Sunday, as scheduled, when 73 players report for the first week of training camp.

Beathard said he hoped to contact Kilmer for more contract discussions before camp starts. Kilmer, currently on the option year of a contract that pays him $220,000 a year, has asked for a guaranteed two-year contract that would provide him "security" for the rest of his career.

Thomas signed a one-year contract midway through the preseason last year after a brief holdout. Under terms of the pact, he was paid $125,000 for the 1977 season. He will receive an automatic 10 percent increase by playing out the option.

Thomas was represented in last year's negotiations by Washington agent Guy Draper, but he switched to the Texas-based firm Athletic Management Associates last spring. Dickens declined to discuss salary demands, but said his client was seeking a multiyear pact "with some guarantees."

"I feel very strongly about having some injury protection in Mike's contract," Dickens said. "He had a big responsibility with the club last year and I assume that he will carry the ball as much this year. Running backs can get hurt, and we wanted Mike protected."

The Redskins will have the option of retaining Thomas after this season if they decide to give him a qualifying offer and then match any offer Thomas receives from another NFL team next spring.

If the Redskins choose not to match an offer, they would be compensated in draft choices by Thomas' new team. Thomas' $125,000 salary would qualify the Redskins to receive first and second round draft choices in the 1979 draft. At the moment, the Redskins do not have a draft choice until the seventh round in 1979.

Dickens said Thomas intends to report to the Redskin training camp as scheduled July 23 with the veterans. "We don't believe in holding people out of camp," Dickens said. "He's in a good frame of mind and he wants to have a great year because it will help his bargaining position."

A year ago, Thomas held out for two days, camp to camp and pulled a hamstring muscle in his leg 20 minutes into his first practice and the injury plagued him all season. He gained 806 yards and scored five touchdowns, down considerably from 1976 when he picked up 1,101 yards and scored nine touchdowns.