LONDON—Prime Minister Theresa May said the U.K. is kicking out 23 Russian diplomats, its single biggest expulsion in more than three decades, and described Moscow as showing complete disdain for the gravity of the use of a nerve agent on British soil.

In the most dramatic diplomatic confrontation between London and Moscow since the end of the Cold War, Mrs. May on Wednesday said the Russian state was culpable for the attempted murder of Sergei Skripal, a 66-year-old former Russian spy, and his daughter Yulia, 33, earlier this...