Anthony Joshua's Aug. 12 rematch with Dillian Whyte in London is off after the Voluntary Anti-Doping Association informed Matchroom Boxing that Whyte "returned adverse analytical findings as part of a random anti-doping protocol," the promoter announced on Saturday.

Eddie Hearn's Matchroom Boxing is attempting to secure a replacement opponent for Joshua on one week's notice, sources told ESPN.

Joshua, the former two-time unified heavyweight champion, was set to fight Whyte in a return bout nearly eight years later. When they met in December 2015, Joshua defeated Whyte via seventh-round TKO but was forced to survive some rocky moments early on against his fellow Englishman.

BREAKING..more updates to follow shortly. pic.twitter.com/cnKFqYgIVV

Eddie Hearn (@EddieHearn) August 5, 2023

Olympic bronze medalist Filip Hrgovic, who is set to fight Demsey McKean on the undercard, is the most attractive replacement option for Joshua. Another possibility is British gatekeeper Derek Chisora, who is slated to fight Gerald Washington on the undercard.

Joshua (25-3, 22 KOs) scored a wide-points victory over Jermaine Franklin in his first fight with Derrick James, ESPN's 2022 Trainer of the Year, leading his corner. The victory came five months after Whyte scored a controversial decision victory over Franklin.

Now training in Dallas, Joshua, 33, is looking to become a three-time heavyweight champion.

An Olympic gold medalist, Joshua lost his three titles to Andy Ruiz in June 2019 in one of the biggest upsets in heavyweight championship history.

AJ regained his titles in December 2019 with a decision victory over Ruiz in the rematch and made a successful defense of the unified championship with a ninth-round KO of Kubrat Pulev.

However, Joshua lost his titles a second time when he met former undisputed cruiserweight champion Oleksandr Usyk in September 2021. Usyk outpointed Joshua again in the August 2022 rematch.

Joshua, ESPN's No. 4 heavyweight, parted ways with longtime trainer Rob McCracken following the first Usyk loss and then ended his partnership with Robert Garcia after only one fight.

This is the third time Whyte has found himself embroiled in a controversy surrounding accusations of performance-enhancing-drug use. Following a victory in October 2012, it was found Whyte tested positive for a banned stimulant. He subsequently received a two-year ban.

Whyte returned to in-ring competition in November 2014. He went on to establish himself as bona fide heavyweight contender with quality wins over Chisora and former champion Joseph Parker. But after Whyte's victory over Oscar Rivas in July 2019, he once again was facing PED charges.

However, UK Anti-Doping later withdrew the charge and cleared Whyte of any wrongdoing. UKAD determined that the metabolite levels contained in that sample were "consistent with an isolated contamination event, and they are not suggestive of doping."

Whyte (29-3, 19 KOs) was stopped in Round 6 by Tyson Fury in his long-awaited first title shot last April at London's Wembley Stadium. Prior to the Fury loss, Whyte, a 36-year-old Londoner, scored a fourth-round TKO of Alexander Povetkin in March 2021 to avenge a fifth-round KO defeat in August 2020.

Whyte is ESPN's No. 7 heavyweight.