Anthony Joshua and Tyson Fury have reached an agreement to fight twice with the first bout to be held next year, according to promoter Eddie Hearn.
Joshua regained his WBA, IBF and WBO heavyweight titles last December by outpointing Andy Ruiz in their rematch while Fury dethroned Deontay Wilder in February to become WBC champion.
The British pair now hold all four major belts in boxing's blue-riband division and speculation of a blockbuster bout to determine the undisputed world heavyweight champion has been steadily building in recent months.
And while Hearn insists there are several obstacles still to overcome, with no contracts signed, Joshua's promoter revealed the fighters are on the same page with regards to the financial terms of both bouts.
"We're making great progress," Hearn told Sky Sports News. "There is still a lot to overcome. We are looking at venues and dates. It's fair to say (Joshua and Fury) are in agreement regarding the financial terms of the fight.
"The most difficult part of any deal is the financial element. I believe we're in a great place where both guys have agreed to what that should be.
"The main positive news is that Joshua and Fury have agreed to a two-fight deal, in essence. The point of Fury, Joshua and the teams agreeing to the structure of the deal? The first fight could happen next summer. It will be 2021."
Joshua must first overcome mandatory challenger Kubrat Pulev when boxing gets the green light to resume properly after all major shows were scrapped because of the coronavirus pandemic.
Fury, meanwhile, is contractually obliged to face Wilder for a third time and the Briton said on Instagram: "The biggest fight in British boxing history has just been agreed. Two-fight deal. Tyson Fury versus Anthony Joshua next year.
"I've got to smash Deontay Wilder's face right in in my next fight and then we go into the Joshua fight next year."
The situation could be complicated further by Dillian Whyte, who the WBC has said must fight for its title before the end of February next year.
Hearn added: "We have not signed contracts because there are still things to be worked out. We're pushing towards a place where they can be drafted, for 2021.
"We've been talking to (Fury's management team) MTK, giving them the assurances from Joshua's side that all the details on the structure of the deal is approved from our side. And it is from Fury's side, as well.
"We have the Dillian Whyte mandatory which is due before this fight.
"(But) both guys are in agreement. The structure of the deal has been put forward, and agreed to by both parties. There is a model in place that both parties are happy with.
"It's the biggest fight ever in British boxing. It doesn't get bigger, and there will never be a bigger fight in our generation."
Frank Warren, one of Fury's promoters, was cautiously optimistic about how negotiations are progressing.
He said in a statement: "Both sides do accept that any prospective fight will demand a 50/50 split and both sides accept there will be a rematch.
"Things are going in the right direction and we couldn't be more happy that a potential mega fight is in reach."