• The CDC said Wednesday that it requested the Justice Department to proceed with an appeal of a ruling that lifted a travel mask mandate.
• The Department of Justice said in a statement that it filed a notice of appeal in the case involving Health Freedom Defense Fund Inc., et al., versus Biden, et al.
• On Monday, U.S. District Judge Kathryn Kimball Mizelle ruled in that case that the travel mask mandate was unlawful.

WASHINGTON — The U.S. Centers for Disease Control and Prevention said Wednesday that it requested the Justice Department to proceed with an appeal of a ruling that lifted a travel mask mandate.

"CDC believes this is a lawful order, well within CDC's legal authority to protect public health. CDC continues to recommend that people wear masks in all indoor public transportation settings," the agency wrote in a statement adding that it will continue to monitor public health conditions to "determine whether such an order remains necessary."

The Department of Justice said in a statement that it filed a notice of appeal in the case involving Health Freedom Defense Fund, Inc., et al., versus Biden, et al.

On Monday, U.S. District Judge Kathryn Kimball Mizelle ruled in that case that the mask mandate on planes and other forms of public transportation was unlawful. The CDC's mask mandate, which was put in place to mitigate the spread of Covid-19, was enacted in February 2021 and extended to May 3.

The new appeal is largely expected to have no immediate effect given that the Justice Department has not yet made an attempt to block Mizelle's order. The appeal process is slated to unfold over a number of months.

On the heels of Mizelle's decision, the White House said that it will likely appeal the decision but that the Transportation Security Administration will not enforce the order on public transport while the ruling is reviewed.

Read more: Faces show relief, confusion and disappointment as masks come off on planes

Some transportation companies, such as the airlines United and Delta and the railroad operator Amtrak, were quick to announce Monday evening that wearing masks was now optional for passengers and employees using their travel services.

If the mandate is reinstated it raises questions about enforcement. The FAA received record numbers of reports of unruly passengers last year, 70% of them tied to disputes over the mask mandate.

Airlines including Delta, Alaska and United said this week that they would start allowing travelers they had banned for not complying with mask mandate back on flights on a case-by-case basis.

"Any further disregard for the policies that keep us all safe will result in placement on Delta's permanent no-fly list," Delta said in a statement late Wednesday. "Customers who demonstrated egregious behavior and are already on the permanent no-fly list remain barred from flying with Delta."

CNBC's Kevin Breuninger and Leslie Josephs contributed to this report from New York.