(Adds details, background on contractor policy)

By David Shepardson and Nandita Bose

WASHINGTON, Nov 1 (Reuters) - The Biden administration said on Monday that a planned rule requiring private-sector employers with 100 or more employees to mandate COVID-19 vaccines or regular testing will be published in the coming days.

The Labor Department said the White House Office of Management and Budget had completed its regulatory review of the rule known as an emergency temporary standard (ETS). The White House said in September the rule would cover more than 80 million private-sector employees.

When the administration will mandate compliance with the new rules is not yet known. Many industries have called for the mandates to be delayed until after the holiday season.

The ETS process was last used in June to institute healthcare workplace rules to stop the spread of the coronavirus. Before that, The Occupational Safety and Health Administration's (OSHA) last ETS was 38 years ago https://www.reuters.com/legal/government/biden-vaccine-mandate-will-test-us-workplace-regulator-2021-09-13/#:~:text=With%20more%20than%2080%20million%20workers%20covered%2C%20%22OSHA,the%20new%20requirements%2C%20the%20White%20House%20said%20Friday and the government has had a mixed record of success using the fast-track procedure.

The ETS has only been used 10 times in OSHA's 50-year history. Courts have invalidated or halted four of those rules and partially blocked one, according to the Congressional Research Service.

Separately, the White House issued new guidance https://www.saferfederalworkforce.gov/faq/contractors on Monday to federal contractors after it said last week https://www.reuters.com/world/us/white-house-signals-flexibility-over-dec-8-vaccine-deadline-businesses-2021-10-27 it was giving them new "flexibility" to meet a Dec. 8 deadline for employees to be vaccinated.

White House COVID-19 coordinator Jeff Zients said he does not expect any disruptions to the U.S. economy or holiday travel as a result of the mandate.

"We’re creating flexibility within the system. There is not a cliff here," Zients said last week.

The mandate covers millions of federal contractor employees and companies including Boeing, American Airlines , Union Pacific and IBM have said they will comply with the contractor mandate.

The new guidance said contractors "should determine the appropriate means of enforcement" for employees refusing vaccination who do not have a pending request for accommodation, such as a religious or medical exemption.

The White House said contractors can follow the federal model to address unvaccinated employees that "encourages compliance, including through a limited period of counseling and education, followed by additional disciplinary measures if necessary. Removal occurs only after continued noncompliance."

Companies can also use "its usual processes for enforcement of workplace policies," it said

The White House said separately that federal employees "should not be placed on administrative leave while the agency is pursuing an adverse action for refusal to be vaccinated."

Federal contractor employees not complying must follow workplace safety protocols for unvaccinated people. Federal agencies can deny entry to unvaccinated contractors to federal workplaces. (Reporting by Nandita Bose and David Shepardson; Editing by Chizu Nomiyama and Bill Berkrot)