LONDON: Calling Russia “a malign force around the world,” Britain’s foreign secretary vowed on Tuesday to retaliate if investigators find that Moscow is behind the apparent poisoning of a former Russian intelligence officer and his daughter in southern England.

The foreign secretary, Boris Johnson, cautioned that it was still not clear who, if anyone, was responsible. But, speaking in the House of Commons, he noted the widespread speculation that Russia was to blame and “the echoes of the death of Alexander Litvinenko in 2006.” Litvinenko, another former Russian agent, was fatally poisoned in London, and a British investigation concluded that he had been killed on orders from the Kremlin.

“Should evidence emerge that implies state responsibility, then Her Majesty’s government will respond appropriately and robustly,” said Johnson, who confirmed the identities of the victims, Sergei V. Skripal, 66, and his daughter, Yulia Skripal, 33. “Though I am not now pointing fingers, I say to governments around the world that no attempt to take innocent life on UK soil will go unsanctioned and unpunished.”

Skripal and his daughter, who became ill on Sunday, were in critical condition after being found on a bench in Salisbury, about 135km southwest of London. The police said they had suffered “exposure to an unknown substance.”

With its echoes of stranger-than-fiction plots from the Cold War and earlier episodes from the Putin era, the case threatens to worsen the already tense relations between the West and a Russian government that has annexed part of Ukraine, propped up the Assad government in Syria and is accused of disrupting elections and sowing discord within democracies.