LONDON — A former Russian spy and his daughter were poisoned by a nerve agent in England this week, the British authorities said on Wednesday, heightening suspicions that the episode was an assassination attempt by a national government, amid rampant speculation that Russia was responsible.
The development forces the British government to confront the possibility that once again, an attack on British soil was carried out by the government of President Vladimir V. Putin, which Western intelligence officials say has, with alarming frequency, ordered the killing of people who have crossed it.
In 2006, Alexander Litvinenko, a former Russian agent who was harshly critical of Mr. Putin, was fatally poisoned in London with a rare radioactive metal; in 2016, an official inquiry concluded that he was murdered by Russian operatives, probably with Mr. Putin’s approval.
“This is being treated as a major incident involving attempted murder by administration of a nerve agent,” said Mark Rowley, Britain’s chief police official for counterterrorism and international security.
The former spy, Sergei Skripal, 66, and his daughter, Yulia, 33, “were targeted specifically,” he said.
Mr. Skripal, a former colonel in Russian military intelligence, and Ms. Skripal suddenly became severely ill on Sunday in Salisbury and remain in critical condition. Some of the emergency workers who went to the scene where they were found also took ill, and one police officer “is now also in a serious condition in hospital,” Mr. Rowley said.