On Thursday, the IBF ordered RING heavyweight champion Wladimir Klitschko to defend its belt against unbeaten Kubrat Pulev, organization Championships Chairman Lindsey Tucker informed RingTV.com.

"Pulev is the mandatory challenger. He won in a box-off that we had and, really, he beat two guys. He beat [Alexander] Ustinov for the number two spot and then he beat Tony Thompson for the number two spot, so he's our mandatory," said Tucker, referring to Pulev's 11th-round knockout of Ustinov in September 2012 and his unanimous decision over Thompson in August 2013.

"But because Klitschko is the unified champion, we have to wait to go into the rotation. But our mandatory was definitely due some time ago. So now, we sent the letter out to get them to start negotiating to get this fight going. They have 30 days to negotiate and if they can't get the deal done within the next 30 days, then the next step is to call a purse bid. They have 30 days and then on the 31st day, the letter would go out calling for the purse bid."

Tucker sent the notifications to Sauerland Events, which promotes Pulev, and K2 Promotions, which has Klitschko.

"On June 6, if they haven't reached a deal, then we would call for a purse bid. Now sometimes, there are requests from both sides to give them a little more time to reach a deal,"said Tucker. "We would give them another 10 days, so that would probably be June 16. Assuming they don't reach a deal in 30 days, we would probably give them another 11 days until that Tuesday, June 17. So the purse bid would be on June 17, and they can have time to reach a deal up until June 16."

Pulev's promoter said he does not believe negotiations will be an issue.

"The negotiations with Team Klitschko have already begun," said Kalle Sauerland. "We are excited to present a heavyweight bout in cooperation with K2, which is deserving of its heavyweight world championship status."

The 6-foot-4 Pulev (20-0, 11 knockouts), 33, was last in the ring for his second consecutive stoppage victory, a third-round knockout of Ivica Perkovic on April 5.

The 6-6 Klitschko (62-3, 52 KOs), 38, made his 16th defense with last month's fifth-round knockout against Alex Leapai, whom he dropped once in the first round and twice in the fifth. Klitschko also holds the WBA and WBO titles.

"That's what I call a late birthday gift. I was ringside when Wladimir defended his belts a few weeks ago. It was a walk in the park for him against Leapai," said Pulev. "When Klitschko meets me inside the ring, I will cause him real problems to say the least. I will show the fans worldwide that Klitschko is no invincible machine and fulfill my dream."