Theresa May, the British prime minister, has accused Russia of being responsible for the “indiscriminate and reckless” nerve agent attack in the English cathedral city of Salisbury that left a Russian double agent and his daughter critically ill.

In a dramatic House of Commons statement in which she threatened robust reprisals against the Kremlin, Mrs May said the former spy, Sergei Skripal, was attacked using a military-grade nerve agent from a family of chemical weapons called Novichok, which were developed in Russia.

“The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” Mrs May said, citing more than a week’s worth of investigations by police at the crime scene and biological experts at the British military’s Porton Down chemical weapons research centre.