In a statement to the House of Commons, May said Russia acted with “complete disdain” and accused it of an “unlawful use of state force”. She has also vowed a response. But what, realistically, can she do?
One classic response is to expel diplomats, and the prime minister did indeed say that 23 Russian diplomats identified as "undeclared intelligence officers" have been told to leave the UK. High-level contacts with Russia have also been cancelled, and government ministers and representatives no longer plan to attend the World Cup this summer.
The UK is also reaching out to its allies. Nato has expressed its "deep concern" at the Salisbury poisoning and said it was a "clear breach of international norms and agreements". And Donald Trump has backed the UK's call for more answers from Russia.
Those are measures we know about. But the prime minister also said that other actions may be taken against Russia. In the days since the attack, several unnamed government sources have suggested a cyberattack could be launched in retaliation.
Is this possible? "For the UK it's not about whether we could launch a cyber action or not," says Beyza Unal, a research fellow at Chatham House's international security department. "They could do it."
Very little is known about the UK's abilities to launch cyberattacks but in November 2016 the Ministry of Defence and GCHQ publicly admitted a National Offensive Cyber Programme exists. Robert Hannigan, the former director of GCHQ, recently said in an interview with WIRED that the nation has "pretty sophisticated" tools at its disposal.
There are no official statistics on how many times the cyber offensive capabilities of the UK have been used. However, the government has said they were deployed against Isis in northern Iraq. It hasn't provided any details of how they were used or whether they were successful.
Unal says it is highly unlikely the UK would launch a cyberattack against Russia. However, if one did take place there could be a number of forms, including disruption of the country's media outlets. (The UK's communications regulator Ofcom is investigating Russia Today's licence to broadcast).
More "creatively," Unal says, the UK could find a way to manipulate the country's internal propaganda. During the Cold War, the US launched the Voice of America radio service, a Russian-language broadcast throughout the country. This was quickly blocked by Russian authorities but a modern equivalent could be created.
Russia currently uses a form of basic internet filtering to police the web within its borders. Unal says: "Is there a way to actually take out that filter through cyber means and open the society to the internet?"
Both Unal and Tim Stevens, a lecturer in global security at Kings College London, agree that a cyberattack against Russia would be an over-reaction. An attack on Russia's critical national infrastructure – such as power supplies – would be disproportionate.
Any attack from the UK could also escalate the situation. Russia has a history of launching cyberattacks after international incidents. Following a diplomatic spat with Estonia in 2007, Russian IP addresses were blamed for taking out online banking and disabling cash machines.
The suggestions the UK could retaliate using cyberwarfare come at a time when tensions between the two country's are strained. In January, the head of the British army said Russia posed the biggest threat since the Cold War. "It will start with something we don’t expect," general Nick Carter said in a 40-minute speech at the Royal United Services Institute. Michael Fallon, the UK's former defence secretary, has also said Western countries need to "deal with" Russia and its war games.
It is possible that the comments from within the UK government have been made to act as deterrent. "You want to be maintaining the moral and legal high ground," Stevens says, saying the case for offensive cyberattacks hasn't been made. "Embarking on a disproportionate cyber response is going to lose you that moral authority."
Stevens adds that it is likely GCHQ and other British intelligence agencies may use more discreet methods of letting their Russian counterparts know what they are capable of. "The intelligence services when they're conducting operations against adversaries are doing all manner of things to signal covertly to their adversary that they know their way around the networks, know where the weak spots are, and so on," he says. "That's a murky world that's highly classified but there will be various forms of covert signalling going on".
Any cyberattack on Russia would be likely to add further tension to provoke the nation. Russian-sponsored hacking groups have advanced capabilities and have shown them in the real-world. Governments in the UK and US have blamed the country for launching the highly destructive NotPetya cyberattack, which disrupted international shipping and shutdown Dutch delivery company TNT. Putin's nation has also widely been blamed for cyberattacks that shutdown powergrids in Ukraine.
Almost immediately after suggestions of a cyberattack from the UK, Russian officials warned that it was ready to hit back. "Not only is Russia groundlessly and provocatively accused of the Salisbury incident, but apparently, plans are being developed in the UK to strike Russia with cyber weapons," a statement from the Russian Embassy in the UK says. "We invite the British side to once again consider the consequences of such a reckless move."