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…

“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.