Anthony Joshua will try to regain his lost heavyweight titles in a rematch with newly crowned world champion Andy Ruiz Jr. in November or December at a location to be decided, promoter Eddie Hearn said on Tuesday.
Ruiz produced one of boxing's biggest upsets by beating the 29-year-old Briton with a seventh-round stoppage at New York's Madison Square Garden on Saturday.
Joshua, previously undefeated and fighting for the first time in the United States, was defending his IBF, WBA, IBO and WBO titles but was knocked down four times in the fight.
"We have today triggered the contracted rematch clause with Andy Ruiz Jnr," Hearn said on Twitter.
"The fight will take place in Nov/Dec at a venue to be confirmed shortly."
Hearn told reporters after the fight that a rematch was a 'must win' for the 2012 Olympic super-heavyweight champion.
"He (AJ) is a competitor, he's meticulous and he has a great mindset. I hope the country gets behind him because this is going to be one of the great comeback stories if he can come back and put this right and beat Andy Ruiz in a rematch," Hearn said to Sky Sports.
"All four belts that were available on Saturday will be on the line for the rematch. Perhaps we'll do the rematch in the UK - nothing has been decided yet, but it looks like November will be the month to make that happen.
"It now turns out to be the biggest fight in the division this year but the pressure on AJ to win this fight is huge. He must win this fight - and he'll want to win this fight more than anything."