Anas Sarwar has claimed he is the victim of a left-wing plot to single him out for sending his children to private school.
The Scottish Labour leadership hopeful said he was being unfairly targeted when other senior figures escaped criticism, including Jeremy Corbyn’s close ally Diane Abbott.
In a hustings at the Brighton conference, both candidates backed stripping private schools of charitable status.
Richard Leonard, Mr Sarwar’s only rival for the role and also privately-educated, went further by calling for independent schools to be forced to pay business rates.
Mr Sarwar, who has faced repeated criticism over his family and business life, told activists: “I should stress that I’m not the only senior Labour politician to send their children to private school.
“I’m not sure why it only becomes a question asked of me and not of the wider Labour movement. Perhaps it’s the faction people are in that depends on that question.”
Mr Sarwar, who is a centrist and backed Mr Corbyn’s opponent in last year’s UK leadership challenge, may have been referring to Ms Abbott, the Shadow Home Secretary, who sent her son to private school.
In what will be seen as a reference to the close relationship between Leonard and Corbyn, the Glasgow MSP warned against Scottish Labour being in the pocket of London.
He said the Holyrood party should not set out to “pick fights with Jeremy Corbyn” but said it was important to have an “autonomous Scottish Labour party” that is “not a branch office”.
Mr Leonard said it would be “unfair to label me a Corbynista” but in a coded swipe at his Blairite rival he said Labour cannot carry on with “business as usual”.
Earlier, Mr Sarwar called for the “spirit of the rules” to be respected, amid claims there could be a trade union stitch-up in favour of his opponent.
It was reported that Unite, which backs Mr Leonard, had signed up 2,700 members to the union’s political levy – a payment that makes them eligible to vote in the leadership contest – in just two days last weekend.
Mr Leonard said the rules of the leadership contest had been agreed and it was wrong to “call foul” after the decisions had been taken.
Both sides have until October 9 to sign up new members or registered supporters, who will be eligible to vote in the contest.