RUSSIA should “go away and shut up," Gavin Williamson has insisted, as Moscow warned the UK it would shortly be expelling a raft of British diplomats from its country.

As Theresa May visited the site of the chemical attack in Salisbury, the Defence Secretary, following his first major speech since taking up the role in November, called for the “whole country to unite" behind the Prime Minister in challenging Russia over its “absolutely atrocious and outrageous” attack in Salisbury.

Asked about retaliation from the Kremlin, he said: "What we will do is we will look at how Russia responds to what we have done…Frankly, Russia should go away and should shut up."

Mr Williamson suggested the Defence and Security Review that pushed through significant cuts to the military in 2010 was based on a "naive" assessment that the threat from nation states had disappeared.

"The whole field of warfare and conflict is changing," he declared.

Russia, argued the Secretary of State, was now operating in a variety of ways and was not holding back.

“You see what they are doing in Syria, what they are doing in Ukraine. They are effectively invading independent nation states and not caring about international reactions.

"We should care about this. We should be bothered about how nations are operating. Britain has always had an amazing position in the world; we actually care about other nations and we care about our security.”

Mr Williamson, who is fighting to protect the Ministry of Defence’s budget, added: "We have got to open our eyes to the fact that people are willing to do so much more than they ever did in the past and we have got to be ready for those threats."

In Moscow, Sergei Lavrov said that, following what Whitehall called the removal from Britain of 23 of Russia’s “undisclosed foreign agents,” tit-for-tat expulsions would "definitely" happen. "Soon. I promise you that," he told reporters.

Russia’s Foreign Ministry, which has denied any state involvement in the Salisbury attack, called the allegations by Mrs May "insane". It claimed the UK's position was irresponsible and not backed up by evidence.

After the US threw its full diplomatic weight behind the UK, denouncing “Russia’s crime” in Salisbury and stressing how it "stands in solidarity with its closest ally," France issued a fresh statement, saying it “shares the assessment of the United Kingdom that there is no other plausible explanation[to Russian involvement] and expresses once again its solidarity regarding its ally”.

The remarks from the Elysee Palace were in sharp contrast with earlier ones from Benjamin Griveaux, President Emmanuel Macron’s spokesman, who denounced Mrs May’s actions against Moscow as “fantasy politics” and suggested France needed proof of the chemical attack before any decisions could be taken.

Boris Johnson claimed Vladimir Putin – expected to be re-elected President in a poll on Sunday - by his government’s actions wanted to send a message to any defecting Russians: “You're going to die."

The Foreign Secretary insisted the actions the UK Government had taken was “commensurate” with what had happened in Salisbury.

"A nerve agent has been used in a European country for the first time since the Second World War. We believe the UK Government has responded robustly as the people of this country would expect us to respond. That should conclude the matter but, of course, the Russian Government may think otherwise," he said.

Mr Johnson told BBC Breakfast: "There's very little doubt in people's minds that this is a signature act by the Russian state, deliberately using Novichok - a nerve agent developed by Russia - to punish a Russian defector, as they would see it, in the run-up to Vladimir Putin's election.

"There's a global disgust at what has happened. We will continue to make the case to our friends and allies that as a comity of nations we need to stand up to Russia."

Mr Johnson said Sergei Skripal, the 66-year-old former spy, had been "singled out already by the Russian state as an object for revenge and retaliation", with the Russian President going on television to say that such people "’deserve to be poisoned, to choke on their own 30 pieces of silver’".

"I'm afraid the evidence is overwhelming that it is Russia," declared the Secretary of State. "And there is something in the kind of smug, sarcastic response that we've heard from the Russians that to me betokens and indicates their fundamental guilt.

"They want simultaneously to deny it and yet at the same time to glory in it. The reason they've chosen this nerve agent is to show that it's Russia, and to show people in their agencies who might think of defecting or of supporting another way of life, of believing in an alternative set of values, that Russia will take revenge.

"That is fundamentally what this is all about. At a time when Russia is going in the wrong direction, becoming more repressive, when Vladimir Putin's regime becomes more corrupt, it's more important than ever for him that he jams down the lid on potential dissent, on potential defectors.

"This is a way of saying to people: 'Look at what happens to people who stand up to our regime,'" added Mr Johnson.

Meanwhile, Downing Street said the Government would seek to hand over a sample of the nerve agent used in Salisbury to the chemical weapons watchdog, the Organisation for Prohibition of Chemical Weapons, “as soon as possible”.

In Salisbury, Mrs May will also receive a briefing from Public Health England.

Home Secretary Amber Rudd is chairing a meeting of the Government's Cobra emergencies committee in London to discuss the latest situation.

And Environment Secretary Michael Gove has chaired a cross-governmental ministerial recovery group looking at support which will go to the people and city of Salisbury in the aftermath of the incident.

The PM's spokesman said subjects discussed included the clean-up of affected areas. The issue of compensation to businesses would be "looked at in the usual way", he added.