PBS’s “Masterpiece,” the enduring Sunday night British drama and mystery franchise, will expand by at least 20 hours next year, an approximate 50 percent increase, with the new hours being broadcast either before or after the program’s traditional time slot of 9 p.m. Eastern time.

The new hours start on Jan. 18, when the six-part mystery “Grantchester” will be broadcast after new episodes of “Downton Abbey.” “Wolf Hall,” adapted from two Hilary Mantel novels, follows in April, with other imports later in the year. They come with exclusive corporate sponsorship from Viking River Cruises, and are a result of the booming interest in the genre by United States viewers.

Led by “Downton Abbey,” the audience for “Masterpiece” has more than doubled since 2010. In 2014, new episodes of “Masterpiece,” through September, are drawing an average 6.4 million viewers weekly; for the same period in 2010, new episodes averaged 2.8 million viewers, “Masterpiece” said.

“How often does this happen to a television series that’s 43 years old, to grow, to be expanded?” said Rebecca Eaton, the program’s executive producer.