Liberal lord mayor Robert Doyle has struck a deal with some of the nation's toughest union bosses to stop a so-called "green ban" derailing a $250 million overhaul of Queen Victoria Market.

The contentious proposal to give the 130-year old market a facelift has become a key issue in the city council election battle, with critics claiming it would ruin the heritage "vibe" of the site and leave stallholders with little certainty over leases.

Concern about the impact has been so palpable that union bosses have threatened to impose work bans to prevent the project, similar to the green bans of the 1970s and '80s credited with saving heritage sites across Melbourne and Sydney.

But in an unusual deal to head off any industrial unrest, Mr Doyle met on October 7 with high-profile union figures, including the boss of the Victorian branch of the Construction, Forestry, Mining and Energy Union John Setka, Electrical Trades Union Victorian secretary Troy Gray, and Plumbers Union boss Earl Setches.