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Britain was tonight preparing to expel dozens of Russian diplomats and spies in retaliation for the deadly nerve agent attack.

As a new cold war loomed, intelligence chiefs and the Foreign Office were drawing up lists of likely candidates to be sent back, on Theresa May’s orders.

The deadly attack – suspected by many to have been ordered by Russian President Vladimir Putin – was “a near-declaration of war” said one source.

And one ex-British intelligence officer told the Daily Mirror: “This could be the largest expulsion of diplomats and Russian spooks in many years.

“It would be a decisive move to show this is being taken extremely seriously.”

Expulsions would be the first in a possible string of punitive measures once the Government is satisfied the Russian state is responsible for the apparent hit on former Russian spy Sergei Skripal and his daughter.

Some believe the outrage could simply be part of Vladimir Putin’s show of strength in the run-up to the first round of elections he is expected to win on March 18.

(Image: Getty Images Europe) (Image: Getty Images Europe)

Other possibilities include a Kremlin faction trying to make trouble for Putin, or a rogue spy settling an old score.

Foreign Secretary Boris Johnson sees the events in Salisbury as “very troubling”.

But he signalled Britain wanted to get to the truth before taking action.

He added: “If this does turn out to be in any way the result of hostile activity by another government, or directed, led, by another government, then the people of this country can be absolutely sure that the UK will respond robustly.”

(Image: Ebru Ozturk)

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Security author and consultant Edward Lucas said: “It’s an extraordinary, audacious choice of victim. It doesn’t just break the rules, it’s an entirely new game.

“Sergei was off the board. He wasn’t in any sense a target — he was living under his own name.

“It means they are taking things to a whole new level — it is in effect a kind of declaration of war.”

Such an assassination bid – including the relative of a spy – is rare, especially when the target has been involved in a spy swap.

And the conflict could broaden out. The ex-British intelligence officer added: “Other allies like America and European countries may be looked at too to take punitive action against Moscow, as this is a grave development.”

It is thought America could consider punitive action against Moscow in unity with Britain and because Skripal was part of a spy swap that also involved Washington.

Former military intelligence Colonel Skripal, now 66, was in the fourth year of a jail sentence for spying for MI6 after being identified by Russian FSB officers as a double agent.

Skripal and three other Russian nationals were pardoned and released in a high-profile spy exchange with the U.S. and Britain.

Experts said tonight that the desire for revenge against a traitor is almost a side-show when it comes to the discord it has caused outside Russia.

It was Putin that passed a law to allow the FSB, formerly the KGB, powers to execute enemies of Russia on foreign soil.

And an assassination attempt like this shows Russia’s extension of power and strength in the same way as Moscow’s bombers, spy planes and submarines have probed Britain’s responses, this also gives the west a prod.

(Image: PA)

The Russian President looked like any other candidate yesterday, posing with female factory workers on the eve of International Women’s day in Samara, central Russia.

But his military parades and nuclear posturing send a clear message.

Speaking in his state of the nation address last week about bitter tensions with the West, he said: “We are a great power, and no one likes competition.”

“But if we play strongly with weak cards, it means the others are just poor players, they aren’t as strong as it seemed, they must be lacking something.”

Mr Putin, who boasted of an array of new nuclear weapons, warned Russia will retaliate in kind if it comes under a nuclear attack.

He added: “The decision to use nuclear weapons can only be made if our early warning system not only detects a missile launch but clearly forecasts its flight path and the time when warheads reach the Russian territory,” he said.

(Image: Reuters)

“If someone makes a decision to destroy Russia, then we have a legitimate right to respond.”

He added starkly: “Yes, it will mean a global catastrophe for mankind, for the entire world. But as a citizen of Russia and the head of Russian state I would ask: What is such a world for, if there were no Russia?”

The former British intelligence officer said: “There are few worse crimes a person can commit against Putin than to give away state secrets to a western intelligence agency.

“And even worse, give it to a British agency, given the so-called special relationship with his nemesis America which is viewed as being at the helm of the threatening west.

“At the same time Skripal is a relatively soft target - as is Britain - and there is a clear message to America, which is that ‘we are having a pop at your closest ally.

“Russia would not dare do this inside America so it may have chosen Britain as the target because the consequences are far less risky.”

Still denying any involvement, Putin and his gangster regime can put up the usual objections about the west wrongly demonising Russia as the scapegoat for other people’s crimes.

(Image: AFP)

This is no small feat in the days before the first round of the Russian presidential elections on March 18 and at a time when nationalists will be looking at Putin for strength.

Shadow Security Minister Nick Thomas-Symonds said: “It is extremely worrying and deeply shocking to read about a police officer who was first to attend the scene being in a serious condition.

“That a police officer doing their duty on a British street has been harmed in this way gives us all pause for thought and shows the gravity of the situation under investigation. “

Commons Home Affairs Committee chairwoman, Labour MP Yvette Cooper, said: “This is a very serious and disturbing development.

“The toxicity of this nerve agent is clear from the fact that the brave police officer who tried to help is now also badly affected.

“For anyone to attempt to murder people in such a vile way is appalling - but the wider questions about who is behind this and how it could happen on British soil are very grave.”

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If Russia was behind the attack, it is virtually unheard of for the Kremlin to kill an agent after a spy swap.

Tonight a spokesman for the Russian Embassy said: “Unfortunately, we have so far received no details on the substance of the case, which is rather worrying.

“Meanwhile, the Foreign Secretary’s strongly anti-Russian statement in Parliament yesterday looks more like an attempt to send the investigation upon a political track.

“Although absolutely no facts were provided to the public, we see the issue being translated into the domain of Russia-UK relations, with an active support by the media.

“The parliamentary debate as well as the Government stance are a testament of London’s growing unpredictability as a partner in international relations, whose policy towards Russia is inconsistent and looks rather miscalculated, not least in the eyes of the Russian public.”