Anthony Joshua's fight against Wladimir Klitschko at Wembley Stadium on 29 April will be watched by the biggest post-war British boxing crowd.
A deal has been struck to ensure 90,000 can attend the fight for Joshua's IBF title and the vacant WBA Super and IBO heavyweight crowns on 29 April.
The Wembley bout will match the British record set by Len Harvey and Jock McAvoy at White City Stadium in 1939.
Crowds are usual capped at 80,000 for transport network capacity reasons.
However, London Mayor Sadiq Khan facilitated an agreement between Network Rail and Transport for London to ensure more services.
"Sadiq urged me to bring the biggest fights to the city and I'm delighted to have the biggest fight in British boxing history at Wembley," said promoter Eddie Hearn.
Hearn also promoted the 2014 rematch between Carl Froch and George Groves, which set the current post-war record of 80,000 and grossed more than £22m.
Joshua has 18 knockouts from as many fights since turning pro in 2013, while Klitschko turned pro in 1996 and has 64 wins (53 KOs) and four defeats.
Klitschko was ringside for Joshua's three-round demolition of Eric Molina at Manchester Arena in December and predicted his Wembley date will be his "signature fight".
The Ukrainian, who turns 41 in March, was unbeaten for 11 years before Manchester's Tyson Fury upset him in Dusseldorf in November 2015.