On 4 March 2018, Sergei Skripal, a former Russian military intelligence officer who acted as a double agent for the UK's intelligence services in the 1990s and early 2000s, and his daughter Yulia were poisoned with a Novichok nerve agent in Salisbury, England. The UK Prime Minister Theresa May requested a Russian explanation by the end of 13 March 2018. She said that the UK Government would "consider in detail the response from the Russian State" and in the event that there was no credible response, the government would "conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom" and measures would follow.