(CNN) In a historic move, President Donald Trump announced Friday that hospitals will be required to disclose the rates they privately negotiate with insurers.

Part of a larger transparency push aimed at reducing health care costs, the controversial requirement has already raised the ire of the powerful industry. Four hospital groups quickly promised to file a legal challenge, arguing the rule exceeds the administration's authority.

The rule is the latest in a series of steps the administration has taken to show what Trump is doing to lower health care costs -- a key concern among Americans and one of his main promises as he heads into the 2020 campaign.

Under the final rule , which stems from an executive order Trump issued this summer, hospitals will have to make public by 2021 the rates they negotiate with insurers and the amounts they are willing to accept in cash for an item or service. In addition, they must provide this information in an online, searchable way for 300 common services, such as X-rays, outpatient visits, Cesarean deliveries and lab tests.

Hospitals that don't comply will face a civil penalty of up to $300 a day.

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