The issue came to light after S35 Community Action Group released a spreadsheet of the councillor’s anonymous comments, posted between 8 May and 15 July.
In one comment, she claimed that the Liberal Democrats blocking the plan would be “disastrous and totally irresponsible”
She said Labour and Green councillors were “actually the ones thinking of the more massive problem Sheffield will have if the Local Plan is delayed or not adopted”.
On other occasions, she responded to critical comments and attempted to defend council leader Tom Hunt, Penistone and Stocksbridge MP Marie Tidball and other councillors.
According to the Local Democracy Reporting Service, Milsom said she had chosen to post anonymously in order to “contribute more freely” and ensure people were “properly equipped with facts”.
When approached about whether any complaint had been made about Milson’s conduct, Sheffield City Council’s Assistant Director of Legal and Governance Robert Parking said: “In line with Council protocol, any complaints must be handled in the strictest confidence and we are not able to comment further.”
A spokesperson for the Labour Party said Milsom was “expected to abide by the Labour whip in future” and said it would be looking into the issue.