All visitors and workers in temples operated by the Church of Jesus Christ of Latter-day Saints should wear face masks “at all times” while in the temple, according to a letter sent by the church’s top leaders to local church leaders around the world on Wednesday.

“We want to do everything possible to allow temples to remain open,” wrote the church’s president, Russell M. Nelson, and his two top counselors, Dallin H. Oaks and Henry B. Eyring. “These safety protocols are temporary, based on Covid-19 conditions, and will be rescinded as soon as circumstances permit.”

Mr. Nelson, 97, is a retired heart surgeon and is revered as a prophet by Latter-day Saints, also known as Mormons. He has repeatedly urged the church’s 16.6 million members around the world to wear masks and get vaccinated. In an August letter, he wrote that the approved vaccines were safe and effective and added: “We can win this war if everyone will follow the wise and thoughtful recommendations of medical experts and government leaders.”

There are 167 temples around the world, according to the church, with dozens more under construction or planned. The church closed them all in March 2020 to help stop the spread of the coronavirus, and began reopening them in phases in May of that year.

Temples are not used by Latter-day Saints for Sunday worship services, and unlike the church’s more common meeting houses, are not open to the general public. They are generally open only to members who have a current “temple recommend,” a card that verifies they believe in the church’s doctrines and obey certain rules such as tithing and abstaining from alcohol. Members visit temples for a variety of rituals, including proxy baptisms for the dead.

“Temples are the most important sacred sites in contemporary Mormonism, so restricting access to temples to people wearing masks sends a big message about how important masks are to the leaders of our church,” said Jana Riess, a senior columnist for Religion News Service who writes about the church.

Although the letter says that the church only “asks” members to wear masks in temples, Ms. Riess said that it is likely to be interpreted as “instruction that’s not to be violated.”

In August, Brigham Young University in Provo, Utah, announced that students and staff members would be required to wear masks on campus. The private school, which is sponsored by the church, also urged students to get vaccinated “so that fall semester can proceed as planned.”

The letter on Wednesday emphasized that there is ample precedent for the church urging members to protect themselves from the spread of the disease. In 1900, church leaders urged members to be vaccinated against smallpox, and in 1957 released a similar message about the polio vaccine.