The pair were set to meet in December only for Casimero to pull out of the fight with viral gastritis - something Butler believed was a cover-up for not being able to make the 118lb weight limit.
The bout was then rescheduled for April 23 but Casimero was blocked from fighting the Englishman for violating BBBofC rules. The Filipino violated safety guidelines for cutting weight in a sauna.
Casimero has now been demoted to number one in the WBO rankings with Butler - who beat Jonas Sultan last month - elevated from interim titleholder to world champion.
He is now a two-time world titlist, although it has not gone down well with Casimero and his manager Egis Klimas.
"This is insane and completely BS, someone here has to put dirty hands on this. First time I am ever seeing fighter being stripped because of using sauna. What's next???," tweeted Klimas, who also looks after Vasiliy Lomachenko.
Casimero also branded it 'insane' and 'unfair' and his former advisor Sean Gibbons - the president of MP Promotions - said 'health and safety of boxing must come first'.
The 32-year-old is a three-division world champion and is on a seven-fight winning streak. His wins include Guillermo Rigondeaux and Zolani Tete but he has not been in the ring since August 2021.
"It is a shame that a great champion like John Riel Casimero made some bad decisions in his career and with his training for his last two fights," Gibbons told Planet Sport about him being stripped of his world title.
"But we all have to live with our decisions and bad advice we take from unqualified people. John Riel broke the rules and that cost him his world title. The health and safety of the boxing must come first."
Gibbons split from Casimero last year after the boxer opted to go back to the Philippines to train rather than remain in America.
He ignored messages and phone calls from Gibbons before severing ties with mentor Noynoy Neri and strength and conditioning coach Memo Heredia. He replaced the pair with his brother Jayson (as head trainer) and Stephen Lunas.
In December, Gibbons spoke to Spin.ph and said he was open to working with Casimero again in the future.
"The relationship is open to be fixed," said Gibbons.
"We did nothing wrong and welcome Mr. Casimero to come back. Mr. Casimero received really bad advice from his ignorant brother Jayson, and adviser Stephen Lunas."