The National Security Council has met to discuss the nerve agent attack in Salisbury on the former Russian military intelligence officer Sergei Skripal, and his daughter Yulia Skripal.
Prime Minister Theresa May told the House of Commons that Russia's ambassador in London has been summoned to the Foreign Office to explain whether the Salisbury attack was "a direct action by the Russian state" or the result of the Russian Government "losing control" of its stock of nerve agents.
Mrs May also said it was clear that Skripal and his daughter were poisoned with a military grade nerve agent of the type developed by Russia.
Earlier, the chairman of the Foreign Affairs committee said that the attack is "looking awfully like it was state-sponsored attempted murder".
Following Mrs May's statement, news agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: "It is a circus show in the British parliament.
"The conclusion is obvious, it's another political information campaign, based on a provocation."
Tom Tugendhat said he would be surprised if Theresa May did not blame the Kremlin.
He told BBC Radio 4's Today programme: "It's a bit too early to be absolutely certain of that but we are expecting to see the Prime Minister make an announcement soon.
"And, frankly, I would be surprised if she did not point the finger at the Kremlin."
Mr Tugendhat warned that football fans travelling to Russia for the World Cup may be at risk of harm if tensions escalate between London and Moscow.
He said: "We do need to be very, very careful for British fans who are travelling there that they are not in any way caught up in the politics of this.
"And, I'm afraid the danger of Russia responding to British fans for actions taken by their government is all too real."
Lord Ricketts, the former UK National Security Adviser, said a boycott of the World Cup by UK officials would not "change the weather in Moscow".
He told BBC Radio 4's Today programme:
"A wide boycott by a number of countries of the World Cup would send a very powerful message that Russia is no longer regarded as a responsible country. I don't know how likely that is, frankly."
The crossbench peer said he hoped there was a "Nato-wide response" if the Kremlin is found to be responsible.
In response to a request for comment from the Press Association, Vladimir Putin's press secretary Dmitry Peskov said: "We consider inappropriate any mention of the Russian government in the context of what happened to Sergei Skripal.
"We have nothing to do with the story."
Meanwhile, the Prime Minister's official spokesman said:
"You have seen words from the PM, Home Secretary and Foreign Secretary and they are very clear that this is an ongoing investigation, that it is important that we allow the police to get on with their work, that we gather all the evidence and if we get to a position when we are able to attribute this attack then we will do so and the Government will deliver an appropriate response."
Those involved include CW specialists from Falcon Squadron of the Royal Tank Regiment; 40 Commando Royal Marines who've recently taken part in Exercise "Toxic Dagger" as well as the RAF Regiment's 20 Force Protection Wing.
Wiltshire Police have told residents "not to be alarmed" as themilitary removes a number of vehicles and objects from in and around Salisbury.
Troops, including Royal Marines, were supported by firefighters, police and medics at Bourne Hill police station in the biggestmilitary operation yet.
Uniformed men put on the now-familiar hazmat suits and gas masks before numbering at least eight marked police vehicles and civilian cars with white spray paint.
Amilitary forklift truck is being used to lift the cars onto the back of low-loader trucks, where they are being covered before the military remove them from the scene.
A week after Sergei Skripal and his daughter Yulia were found suffering the effects of a suspected nerve agent attack, troops and chemical weapons experts have returned to the Wiltshire city for a third consecutive day.
Police investigating the suspected nerve agent attack in Salisbury have identified more than 240 witnesses and 200 pieces of evidence, Home Secretary Amber Rudd said.
Following a meeting of the Government's Cobra emergencies committee on Saturday, Ms Rudd said there were more than 250 officers from eight out of 11 of the country's counter-terrorism units involved in the investigation:
"This is a serious, substantial investigation. We need to give the police and the investigators the space to get on with that.
"I want to stress that they are proceeding with speed and professionalism. We are putting in enormous resources to ensure that they have all the support that they need to do that."
Ms Rudd said Mr Skripal and his daughter Yulia remained in a "critical" condition.
Detective Sergeant Bailey was also still "seriously ill" but had been speaking to his family in hospital.
She said it was still too early to say who was responsible for the attack.
Salisbury's MP John Glen, a Government minister, has reassured his constituents that a "whole range of tools are at our disposal" once it is established who was behind the poisoning of a former Russian spy.
Mr Glen responded to some locals who had demanded "decisive action" over the incident in a Facebook post:
"The Government will not be acting precipitously in response to what has happened. Now is the time for cool heads and a rational examination of the facts.
"Once these are established, then, and only then, will an appropriate and proportionate course of action be taken."
In a statement, the Metropolitan Police said:
"The Counter Terrorism Policing Network has requested assistance from the military to remove a number of vehicles and objects from the scene in Salisbury town centre as they have the necessary capability and expertise.
"The public should not be alarmed and the public health advice remains the same.
"The military has the expertise and capability to respond to a range of contingencies.
"The Ministry of Defence regularly assists the emergency services and local authorities in the UK. Military assistance will continue as necessary during this investigation."
The circumstances of the attack, and its echoes of the fatal poisoning of Alexander Litvinenko in 2006 have prompted questions over the Government's response if the evidence points to a state-sponsored assassination plot.
Security Minister Ben Wallace said the Government was ready to respond with "the full force of the United Kingdom's resources" once they had established who was responsible for the attack.
He told BBC Radio 4's Today programme:
"Once we have established the facts and the attribution, the Government and law enforcement and others will respond appropriately as a country such of the United Kingdom should.
"We will respond with the full force of the United Kingdom's resources if that is the appropriate and proportionate thing to do.
"If we are to take solid steps in response to whoever has done this, we are going to make sure that we do it in a considered (way) - but we also do it in an effective way and in a way that makes sure that Britain and British citizens and their interests are safe as a result of that response.
"We want to make sure that we are effective in whatever our response is. If that is arresting a couple of people and getting them in jail, then that is what we shall do.
"There are lots of things that the United Kingdom can do. It is a powerful country with a powerful economy, powerful allies, powerful military and powerful other capabilities and we shall look at those all."
Russia has denied responsibility for the attack, which comes seven years after Mr Skripal was released from the country as part of a spy swap with the US.
He had been convicted in his home country in 2006 for passing state secrets to MI6.