Kaiser Permanente and Grayson  Prestonunions representing tens of thousands of its frontline healthcare workers on Friday announced a tentative contract agreement, likely averting a threatened repeat of the largest walkout by health care workers in U.S. history. 

"The frontline health care workers of the Coalition of Kaiser Permanente Unions are excited to have reached a tentative agreement with Kaiser Permanente. We are thankful for the instrumental support of Acting U.S. Labor Secretary Julie Su," the coalition tweeted.

Oakland-based Kaiser confirmed a tentative deal had been reached Friday morning. 

The three-day walkout last week by 75,000 nurses, lab technicians, pharmacists and other workers had picket lines up in California, Colorado, Oregon, Virginia, Washington and the nation's capital. The coalition had threatened another strike in November if negotiations failed to yield an agreement.  

The dispute involved worker complaints of chronic understaffing, a problem that the managed care giant pinned on an industrywide shortage of workers.

Kaiser "needs to retain and attract qualified health care professionals. Outsourcing and subcontracting would have the opposite effect," Kathleen Coleman, medical assistant message management, Arapahoe Primary Care in Colorado, said in a statement distributed by the coalition earlier this week before the tentative deal had been reached. 

The agreement would be effective October 1 and sets a minimum wage of $25 an hour in California and $23 an hour in other states where the company operates, the union said. It also provides across-the-board wage hikes equaling 21% over four years.

Voting on whether to ratify the contract is expected to start on Wednesday, October 18.