The Towers, who defeated the Derby Bucks 92-84 on Saturday, are now four points behind Sheffield with each club having four games remaining to play.
Two three-throws from Karl Brown sealed victory for the Leopards, who went into the game without their suspended coach, Billy Mims, and with only one American, following the two-year ban on DeCarlo Deveaux following his involvement in the brawl at Derby two weeks ago.
Brown scored 10 of the Leopards' first 15 points, including three three- pointers, to inspire a victory which gives their London rivals hopes of winning their first championship.
After leading 56-35 at half-time the Towers withstood a strong Derby fightback, capped by a three-pointer from Carl Gonder, which cut London's lead to 84-78. Towers' American, Tony Windless, scored 25 of his 27-point total in the second half.
Doncaster Panthers' hopes of winning the title ended with an upset 98- 80 defeat at Chester Jets, who are second from the bottom of the league. Panthers were not helped by losing their American, Chris Fite, two minutes into the game with an ankle injury.