LONDON — A 44-year-old British woman who was exposed to a nerve agent died on Sunday, bringing new urgency to a four-month-old diplomatic standoff in which Britain has accused Russia of sending the poison to a small city in southern England in a botched attempt to kill a former spy.

The British authorities have now opened a murder investigation.

“Today is the day we hoped would never come,” said Chief Constable Kier Pritchard of the Wiltshire Police.

The police say the woman, Dawn Sturgess, was most likely exposed accidentally to residue from a Soviet-developed, military-grade nerve agent used in a March attack on the former spy, Sergei V. Skripal. He lived near Ms. Sturgess in Salisbury.

After Mr. Skripal and his daughter, Yulia, collapsed, British officials declared confidently that Russia was at fault, in large part because of the obscure poison involved. It was one of a strain of nerve agents known as Novichok that they say is kept under tight control by the Russian authorities.