LOS ANGELES—Los Angeles Times journalists will conduct the first newsroom union work stoppage in the 142-year history of the newspaper Jan. 19, one day after management disclosed that substantial layoffs were coming due to a widening budget deficit.

The one-day unfair labor practices strike was prompted by management’s request to waive seniority provisions in the union’s contract covering layoffs, according to a statement from the Los Angeles Times Guild, the union representing the paper’s newsroom employees.

“The changes to our contract that management is trying to pressure us into accepting are obscene and unsustainable,” Brian Contreras, chair of the Guild’s Unit Council said in a statement. “If this newsroom will ever be a place where reporters can have a reliable, steady job and put down roots in Los Angeles, that will only happen through the preservation of our seniority protections.

“And if management thinks our financial situation is untenable, they need to come to the bargaining table in good faith and work out a buyout plan with us that would first articulate a clear headcount or cost saving they’re aiming for, and then seek to hit that number with as many buyouts—and as few layoffs—as possible.”

Management has not publicly disclosed the number of newsroom positions that will be eliminated, but knowledgeable people said the plan is to lay off at least 100 journalists, or about 20 percent of the newsroom, the largest staff cut since the paper was owned by Tribune Co., The Los Angeles Times reported.

The planned layoffs will mark the third round of cuts since June, when more than 70 positions, or about 13 percent of the newsroom, were trimmed.

“We need to reduce our operating budget going into this year and anticipate layoffs,” Los Angeles Times spokeswoman Hillary Manning said Thursday in a statement. “The hardest decisions to make are those that impact our employees, and we do not come to any such decisions lightly.

“We are continuing to review the revenue projections for this year and taking a very careful look at expenses and what our organization can support.”

The union will have what it is billing as a “Rally To Save Local Journalism,” at noon in Gloria Molina Grand Park. Similar events are planned for Sacramento and Washington.

The union is asking the public not to click on Los Angeles Times stories, engage with the brand’s accounts on social media, or open Los Angeles Times news alerts through 11:59 p.m. Friday.

The Los Angeles Times layoffs follow an extensive trend in print media stemming from reduced advertising and subscription revenue.