Heavyweight world titleholders Anthony Joshua and Joseph Parker are on the verge of a deal to unify their belts this spring, Parker co-promoter David Higgins of Duco Events told ESPN on Wednesday night.

The biggest issue through the more than a month of negotiating the fight -- one of the biggest that can be made in boxing, as it would match two unbeaten fighters for Joshua's two belts and Parker's one -- has been how the sides would split tens of millions of dollars in revenue. But Higgins said he and Joshua promoter Eddie Hearn of Matchroom Boxing have finally reached an agreement on that point, and Hearn has sent him a contract.

"The fight has never been closer. We're a lot closer now to a deal because we've had a major breakthrough, which is that the two camps have formally agreed on the split," Higgins told ESPN in a phone interview from his home country of New Zealand. "The Joshua side made a small concession, and the Parker side has made a small concession."

Initially, Parker's side demanded 40 percent, with Hearn offering a fraction of that figure. Although Higgins declined to give the precise percentage split the sides agreed to, he said it was between roughly two thirds of the pie for the Joshua side and a third for the Parker side.

"It's between 30 and 35-ish percent for the Parker side," Higgins said. "I'll leave it at that for now, but it's somewhere close to the middle of that. Maybe after Eddie and I talk about it, we will make the figure public."

There is no set date or location for the bout, but Higgins acknowledged that it would take place on a Saturday no sooner than March 31 and no later than April 28. As for the location, he said it would take place in United Kingdom or perhaps somewhere else in Europe, "but you never know if a place like Saudi Arabia comes in and offers a $100 million to stage the fight."

Barring some unexpected giant site offer, Higgins said he and Hearn have discussed possible sites in Manchester or London, England, and Cardiff, Wales, as well as "a couple of wild-card venues in Europe because Joshua is so big you could have tens of thousands of fans travel to another location in Europe for the fight."

Higgins said there are still a few smaller items to deal with, but nothing that he believes will derail a deal. He said the contract that Hearn sent him a couple of days ago calls for one ringside judge to be from Joshua's home country of the United Kingdom, one from Parker's home country of New Zealand and one neutral. Higgins said he prefers three neutral judges and a neutral referee.

"My gut feeling is there will be a deal. We're only haggling over officials," he said. "We'll have to have a discussion with the British Boxing Board of Control [which regulates the sport in the U.K.]. I'm also happy to have the sanctioning bodies nominate a pool of officials. But even with that, we're very close to a deal."

Higgins said he and Hearn have also agreed on random drug testing to be overseen by the Voluntary Anti-Doping Association as well as a rematch clause for Joshua, should he lose.

"I think that's fair," Higgins said of the rematch clause. "Should Parker win, there'd be a rematch, and I think that's fair because, commercially, Joshua is the A-side of the fight."

Although there has been a bit of public back-and-forth between Higgins and Hearn during the negotiations, they have been able to get this far on a very big fight with few major issues.

"Eddie and I get on well," Higgins said. "I found him to be smart, and we have a banter and humor between us. He doesn't have an ego problem. I like working with people like Eddie Hearn. Both sides have given concessions. We respect them."

Higgins said he expects to fly to England to meet with Hearn next week to finish the deal.

"The goal is to get it done as soon as possible," Higgins said.

The 28-year-old Joshua (20-0, 20 KOs) is perhaps the biggest global star in boxing, having drawn big pay-per-view numbers in the U.K. and enormous sold-out crowds for his past two fights: 90,000 to Wembley Stadium in London for his epic, 11th-round knockout of former longtime unified world champion Wladimir Klitschko in April and 78,000 to Principality Stadium in Cardiff, Wales, for a 10th-round stoppage of former Parker victim Carlos Takam on Oct. 28. Joshua, the 2012 Olympic super heavyweight gold medalist, has made four title defenses and is the money man of the division.

Parker (24-0, 18 KOs), who turns 26 on Jan. 9, won a vacant world title by decision over Andy Ruiz Jr. in New Zealand in December 2016 and has defended the belt twice. He won a lopsided decision against Razvan Cojanu in New Zealand on May 6, and then he traveled to Manchester, England and won a majority decision against mandatory challenger Hughie Fury, the cousin of former world champion Tyson Fury, in his hometown on Sept. 23.