The Defence Secretary hinted at further measures to punish Russia, as he told Vladimir Putin to “go away and shut up” rather than expel British diplomats in retaliation.
“It is absolutely atrocious and outrageous what Russia did in Salisbury,” he said in his first major speech since his appointment.
“We have responded to that. Frankly, Russia should go away, it should shut up - but if they do respond to the action that we have taken we will consider it carefully and we will look at our options.”
The comment came in answer to a question about whether Britain was considering a “military” response if relations between the two countries continue to deteriorate.
Asked if Britain and Russia were entering a “new Cold War”, Mr Williamson replied: “Relations ain’t good are they?”
And he added: “It is often described as a cool war that we are entering into. I would say it’s feeling exceptionally chilly at the moment.”
Mr Williamson denied he had criticised steep defence cuts since 2010, but said Britain now had to invest in “hard power” to meet the growing threat from Russia and elsewhere.
The assessment, at the start of the decade, that “state-based threats no longer existed” had been “naïve”, he argued.
“There were state-based threats, but they were not as obvious as they are now,” Mr Williamson said. The common belief at end of the Cold War that world would become a “better and safer place” was no longer “the reality".
The speech came as the Prime Minister visited Salisbury to speak to emergency services, members of the public and local businesses and receive a briefing from Public Health England.
Home Secretary Amber Rudd was chairing a meeting of the Government's Cobra emergencies committee in London to discuss the latest situation.
In Moscow, the Russian foreign minister Sergei Lavrov warned the Kremlin will expel British diplomats “soon” after Ms May announced the biggest expulsion of Russian embassy staff since the Cold War.
In his speech, Mr Williamson announced a multimillion-pound investment in a new specialist chemical weapons defence centre at the secret Porton Down facility.
And, after the nerve agent attack on ex-spy Sergei Skripal and his daughter, British troops will also be vaccinated against.
“If we doubted the threat Russia poses to our citizens, we only have to look at the shocking example of their reckless attack in Salisbury,” Mr Williamson said.
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