Anthony Joshua will now face Robert Helenius at the O2 Arena in London on Saturday.
The two-time world heavyweight champion had been scheduled to fight old rival Dillian Whyte in a rematch of their 2015 clash at the same venue.
However, it emerged on Saturday that Whyte had provided an 'adverse analytical finding as part of a random anti-doping protocol' a finding that Whyte strenuously denies.
As it stands, the substance that Whyte tested positive for remains unclear, yet Matchroom Sport were in a position where they faced cancelling their second sold-out O2 crowd in less than 12 months if they could not find a replacement opponent.
While clashes with Dereck Chisora or Gerald Washington, who fight on the undercard, were rumoured, Joshua will instead square off against Helenius.
At short notice, the Finn is a credible opponent as the 39-year-old holds a record of 32 wins and four defeats from 36 outings in the professional ranks.
That said, the most recent of those losses was a first-round knockout against Deontay Wilder, who Joshua is planning on facing if he can prevail at the weekend.
Helenius only made his return after a 10-month absence on Saturday, easing to a third-round win over compatriot Mika Mielonen in their homeland.
Speaking on the newly-scheduled fight, Joshua said: "This wasn't in the script. I respect Helenius and may I say, I respect any male or female who steps into the ring.
"I am laser focused on the win. I can make steps forward to bigger and better things but the road map has a check point, Saturday night. May the best man win."
Commenting to Matchroom Sport's official website, Helenius added: "I am excited about fighting Anthony Joshua on August 12. I am a true Viking that is willing to face any challenge at a moments notice.
"This is not an opportunity I was going to let slip away. I plan to make the most of it!."
Whyte faced Helenius in Cardiff back in 2017, overcoming a slow start to record a resounding points win. body check tags ::
