Shareholder advice group Pirc has called on investors to drop Sports Direct's chairman in a vote on Thursday.
Aberdeen Asset Management has also backed calls for chairman Keith Hellawell to go.
Hellawell faces expulsion after the majority of the retailer's independent shareholders voted to boot him from his role at the company's annual general meeting (AGM) in September.
Sports Direct announced on the day that Hellawell would be staying, but under new rules, shareholders have been given a second vote to decide if he should stay in his post.
In a document issued to shareholders today, Pirc said Hellawell had "failed to show leadership" during a period in which the company had faced a parliamentary inquiry into how it treats its workers. Under Hellawell's stewardship, Sports Direct had "failed to address the employment practices issues raised by some shareholders and trade unions".
The parliamentary inquiry concluded that some of Sports Direct's employees had been subjected to "Victorian" working conditions.
Paul Lee, head of corporate governance at Aberdeen Asset Management, added: "Though we welcome the positive progress the company has made over the last three months we remain deeply concerned about its governance.
"We are therefore again opposing the election of Mr Hellawell."
It is thought that Sports Direct's chief executive Mike Ashley, who is also a controlling shareholder at the company, will use his voting power to save Hellawell.
However, Hellawell promised independent shareholders at the company's AGM that he would leave if they voted for him to go in the new year.