Union boss John Setka’s threats to slow down key Australian Football League projects unless it sacks its umpires chief could be illegal under workplace laws, and may even amount to blackmail, lawyers say.

Workplace Relations Minister Tony Burke on Thursday condemned Mr Setka for personally targeting the AFL’s new head of umpires, Stephen McBurney, saying it gives unions a bad look and warning the CFMEU not to come between Australians and their sport.