Kevin Millar and Chris Rose discuss the Yankees and Red Sox crossing the pond to play a series in London in 2019. (1:40)

Major League Baseball's first game in London, featuring the World Series champion Boston Red Sox and rival New York Yankees, is the best-selling game of the 2019 regular season, ticket seller StubHub said on Thursday.

Following matchups in Mexico and Japan in recent years, the league is making its first foray into Europe with hopes of expanding its international reach with the help of its two most-storied franchises.

Fans from 44 different states in the U.S. and 27 different countries have already purchased tickets for the MLB London series, according to StubHub general manager Jill Krimmel.

"The appeal of MLB to a global audience continues to grow, and the overwhelming response for the rivalry matchup between the Yankees and Red Sox in London certainly proves that the sport has fans worldwide," Krimmel said.

"It is demonstrating that baseball has tremendous potential overseas, much like the NFL and NBA have seen."

The Yankees are driving demand stateside, outselling the Red Sox by 18 percent.

The Philadelphia Phillies have seen a surge in ticket sales following the acquisition of slugger Bryce Harper, who signed a record 13-year, $330 million contract.

Phillies' ticket sales are up 93 percent from the same time last year and up 157 percent over the last three seasons, the highest three-year growth out of all the teams in the league.

MLB's Opening Day is March 28. The London Series takes place at the Olympic Stadium over two days, June 29-30.

Tickets for the London Series sold out in December, but the MLB announced on Thursday that a further 5,000 tickets will be released across the two games due to an increase in capacity at the London Stadium.

The stadium, which was built for the 2012 Olympic Games and is now home to Premier League soccer team West Ham United, will see its capacity increase from 55,000 to around 57,500.

West Ham have averaged a home attendance of 57,902 during the 2018-19 Premier League season.