"Uefa, who I talked to during and after the match, will conduct a thorough investigation to ensure that these nasty scenes of horrible racism are treated properly," Clarke told the BBC.
The chairman, who was present in Sofia, describes what happened.
- I heard examples of horrible racist rams. I saw a group that was about 50 people large and completely dressed in black who did something that looked like fascist gestures. I can't be sure because it was 100 meters away from me, but it looked awful.
Clarke talked to both players and leaders after the match that should have been "clearly upset."
During the match, Uefa's three-step manual was followed when you first interrupted the match in the 28th match to condemn the hopes of the stadium and announce that you will interrupt the match if it continues.
The next time the game was stopped was in the 43rd minute and according to the manual, the team would then have entered the locker rooms to mark. But British captain Gareth Southgate talked to the referee and they arrived to play the few remaining minutes before the break.
- We used our common sense and made the decision to play the few minutes that remained until half time and then we jointly made the decision to continue playing. We talked about it during the break and handled it properly, says player Tyrone Mings.
The match ended 6-0 to England.