The 63-year-old has been criticized for saying more power stations should be built in Africa, “then Africans would stop felling trees and producing children when it gets dark”.
The billionaire businessman made the remark at a forum in Paderborn on Thursday while criticizing tax increases to fight climate change.
On social media, numerous Schalke fans demanded his resignation as chairman of the club's supervisory board, while senior figures in German football and politics have condemned his comment.
Tönnies apologized for his “inappropriate” words on Sunday, insisting he backs Schalke's values against “racism, discrimination and exclusion”.
However, a statement on the club website may not be enough to save him with Germany's Justice Minister Christine Lambrecht calling on the German Football Association (DFB) to “deal” with Tönnies.
“Racism must be loudly and clearly contradicted” at every opportunity,” the politician told the Funke media group.
“Nowhere is integration as successful and quick to work as in sport – that must not be put at risk.”
Tönnies' comments are a distraction for Schalke, under new head coach David Wagner, who start their league season at Borussia Mönchengladbach on August 17.
President of the German Football League (DFL) Reinhard Rauball told news agency DPA Tönnies' comments are “completely incompatible with the values of football” held by both the DFB and DFL.
'Active repentance is needed'
Dagmar Freitag, chairman of the sports committee in the German parliament, says an apology is not enough.
“The fact that something like this is articulated by someone who holds a prominent position in sport makes things all the worse,” Freitag told German newspaper Welt on Sunday.
The politician says an apology alone “certainly cannot make up for the sociopolitical damage”.
Likewise Tönnies must send a “clear signal” to atone, Sylvia Schenck, a lawyer for Transparency International also told Welt.
“Such a thing doesn't just slip out during an official speech, there is a highly problematic attitude behind it”, said the former middle-distance runner.
“Active repentance with clear signals to Africans is necessary in order to really prove Tönnies has changed his opinion,” she added.