The lightweight duo were in talks for a historic four-belt lightweight showdown in Australia this summer.
The two-time Olympic gold medallist has taken up arms to defend his nation with the Belgorod-Dnestrovsky Territorial Defense Battalion according to reports.
It leaves Kambosos Jr without an opponent for his maiden title defence.
However, writing on social media after hearing the news, the Australian star has sent a message of solidarity to Lomachenko.
Kambosos wrote on Twitter: “Vasyl Lomachenko I respect your decision and I totally understand and I pray for you and your country.
“Please stay safe and once I wipe the floor with Devin, we will make this fight between two real champions. God bless.”
Kambosos, aka ‘The Emperor’, beat Teofimo Lopez in a stunning upset to capture the belts back in November.
He is now expected to face Devin Haney in a lightweight unification later this year.
Meanwhile Kambosos’ promoter Lou DiBella has also expressed his admiration for Lomachenko having revealed a deal had already been agreed with Top Rank CEO Bob Arum before Russia invaded Ukraine.
Speaking to ESPN, he said: “How can anyone question, the fight he’s going through right now is much bigger than any boxing match.
“We made a deal with him, the deal was literally done before the invasion, we wanted to give him any chance within reason with our allowable timetable.
“We have nothing but the utmost respect for his decision. Kambosos wanted to make sure once we made that deal that Lomachenko had ample time to decide.
“We said all along we would try to make the biggest fights and we did that today.”
Kambosos’ manager Peter Khan added: “The WBO had made it very clear that it would strip George Kambosos Jr if he were to fight anybody other than the mandatory, Vasiliy Lomacheko.
“George did not want to relinquish any of the belts and therefore was going to fight the mandatory. In his mind he was always going to win and then would move on to fight Devin Haney.
“Yes, there were other factors at play regarding the deals that were presented but it also served another purpose, which is to keep the belts intact.”