Umpires blew a call on a home run ball that helped the slumping Kansas City Royals beat a New York Yankees team battling for top spot in the American League East, Major League Baseball (MLB) said on Thursday.

The botched call that awarded Billy Butler with a go-ahead homer and gave Kansas City a 4-2 lead in the third inning of Wednesday's game was the result of confusion over ground rules in Kansas City, according to MLB.

There really was a misunderstanding about what the ground rule represented, Joe Torre, a former Yankees manager and now vice president in charge of baseball operations for MLB, told reporters at the owners meetings in Cooperstown, New York.

I talked to (umpiring crew chief) Dana DeMuth ... he was very sure that that ball was a home run because it hit that back fence, which he thought was out of the park.

Butler's drive struck the top of the left-field wall, a section of railing that is still in play at the ballpark, and bounced off a chain-link fence and back on to the field.

Even after a video review, DeMuth ruled it was a homer. It should have been a double and might have made a difference in the game.

The Royals, who are well out of contention in the American League Central, went on to a 5-4 win after holding off a rally by the Yankees in the ninth inning.

Yankees manager Joe Girardi initially argued the umpires' decision, but accepted their explanation and did not file a protest which might have led to some form of remedy in light of Torre's comments and an apology later issued by DeMuth.

Girardi said he did not officially protest because both DeMuth and the home plate umpire both reassured him that they clearly understood the stadium's ground rules.

They both were so adamant that that was the rule, that it just had to clear the one wall, Girardi, whose Yankees are in a tight divisional race with the Boston Red Sox, told New York's WFAN radio on Thursday.

In hindsight, I probably should have (protested).