South Africa's main opposition party said on Sunday that it had charged a provincial leader with misconduct after she posted on Twitter that colonialism had brought benefits including clean water.
Helen Zille, former leader of the Democratic Alliance (DA) and the current premier of the Western Cape province, could be expelled for the tweet, which unleashed fierce criticism from her own party and opponents.
She will remain the leader of the Western Cape during the internal party probe.
"For those claiming legacy of colonialism was ONLY negative, think of our independent judiciary, transport infrastructure, piped water," she wrote on Twitter last month.
DA leader Mmusi Maimane said at a press briefing in Cape Town that disciplinary proceedings had begun against Zille, but that she would not be suspended during the investigation.
"Our party has always stood for the principle of freedom of speech. This case is not about freedom of speech. Our party has stood for South Africans from all walks of life," he said.
"People can express a view but does that view do damage or harm to the interests of the organisation, which is what this is."
The DA, which won 22 percent of the vote in 2014's general election, has been gaining popularity as it tries to shed its image as a "white" party.
But efforts to broaden its appeal among black voters have been bruised by social media scandals, and the party has struggled to present itself as a credible alternative to the ruling African National Congress (ANC).
"For Democratic Alliance black members, uncertainty remains on where the party stands on racism and on the treatment of white and powerful leaders," the ANC's Western Cape office said in a statement.
"Maimane has not shown the same passion in putting South Africans first above Helen Zille who singlehandedly put him where he is," it said. "The DA should have suspended Helen Zille as premier."
If the DA upholds the misconduct charge against Zille, sanctions could include temporary suspension, a fine of up to $3,700 (3,500 euros), community service or permanent expulsion from the party.
"There is preliminary evidence that there is a strong case to be made," Maimane said.
"I've got to build an organisation that reflects South Africans. In that instance premier Zille has the right to answer.
"It is personal, I have respect for premier Zille, she has served this country with distinction -- it would be incorrect to suggest that she hasn't.
"It would also be incorrect to draw conclusions to say that she's racist. That's not the person I know."