LONDON — It seems simple enough: If you want a divorce, you can get a divorce, right?

In Britain, it turns out, the answer is “not necessarily.”

On Wednesday, the Supreme Court ruled — grudgingly, even apologetically, but unanimously — that Tini Owens, who has been trying for years to get a divorce, must remain in her unhappy marriage to Hugh Owens, who opposes the divorce.

Ms. Owens’ predicament is not permanent. The law says that if a couple have lived apart for five years, one spouse can get a divorce even if the other objects. Ms. Owens will reach the five-year mark in 2020.

A 1973 law allows divorce on the grounds of adultery, abandonment or “that the respondent has behaved in such a way that the petitioner cannot reasonably be expected to live with the respondent.”