Labour staff have threatened to strike if the party pursues compulsory redundancies.
Keir Starmer was warned by Unite and GMB unions that members could take industrial action if jobs are axed as part of a planned restructure.
Both unions backed action by 75% if compulsory redundancies are announced, with turnouts of approximately 80%.
Another ballot would have to be held before any industrial action is taken.
Labour is aiming to claw back cash through voluntary redundancies, with applications closing on Tuesday.
Get a daily morning politics briefing straight to your inbox. Sign up for the free Mirror Politics newsletter
Unions will meet party officials for crunch talks on Friday in the hope of heading off compulsory job cuts and industrial action.
Unite regional officer Matt Smith said: " Labour will be meeting with Unite and the GMB on Friday and both unions are hopeful that a way forward can be found that avoids any compulsory redundancies or resultant industrial action."
GMB official Vaughan West said: "Labour Party workers have shown the strength of their anger at this ballot result.
"We hope party bosses will now sit up, take notice and talk with unions so we can avoid compulsory redundancies."
The row comes ahead of Mr Starmer's first in-person conference as leader as last year's event was held virtually due to the pandemic.
He has been under pressure to spell out his vision to voters after the loss of the Hartlepool by-election to the Tories and a disappointing set of local election results.
However Mr Starmer was handed a boost by scoring victory in the Batley and Spen by-election in July.
Party conferences under his predecessor Jeremy Corbyn were often marred by factional in-fighting.
Labour has been contacted for comment.
