Video report by ITV News Political Correspondent Romilly Weeks
Russia is prepared to ignore a UK ultimatum over the attempted murder of Sergei Skripal, according to the country's embassy in London.
Theresa May demanded that Moscow give an account for the nerve agent attack on Mr Skripal after the Government concluded it "highly likely" the Kremlin was involved.
But the Russian Embassy in the UK responded by saying Moscow would ignore the ultimatum, which expires on Tuesday, unless it is given access to a sample of the alleged nerve agent, Novichok.
And in a series of tweets, the Russian UK embassy warned that Moscow could retaliate if Britain imposes measures against it.
The warning comes after the US backed Britain over the Salisbury attack and developing diplomatic row.
In a phone call with the prime minister, Donald Trump said the US was "with the UK all the way" and called on Russia to provide "unambiguous" answers, according to Downing Street.
A huge inquiry is continuing after Mr Skripal, 66, and his 33-year-old daughter were found slumped on a bench on March 4. They remain in a critical condition.
Detective Sergeant Nick Bailey, who was part of the initial police response, is in a serious but stable condition.
In other developments:
Police revealed 38 people have been seen by medics since the attack
Home Secretary Amber Rudd announced MI5 and the police are to look into allegations that a string of deaths on UK soil may be linked to Russia
Foreign Secretary Boris Johnson said the UK has been encouraged by the "willingness" of other countries to "show support and solidarity" with Britain
Setting out Moscow's position, Russia's UK embassy said: "Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance to which the UK investigators are referring."
It continued: "Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready.
"Without that, there can be no sense in any statements from London. The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia."
The embassy then reiterated in a tweet that Russia was "not involved".
On Monday, the White House had appeared reluctant to point the finger at Russia over the attack, simply saying it supported the British Government.
But Mr Trump's now former secretary of state Rex Tillerson then said that the nerve agent used in the attack "clearly came from Russia".
Following the phone conversation between Mr Trump and Mrs May, a Downing Street spokesperson said the US president had called on Russia to open up about the attack.
"The Prime Minister set out the conclusion reached by the UK Government that it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal," the No 10 spokesperson said.
"President Trump said the US was with the UK all the way, agreeing that the Russian government must provide unambiguous answers as to how this nerve agent came to be used."
What is the latest from the British Government?
On Monday, Mrs May set a deadline of midnight on Tuesday for the Russians to provide a credible explanation as to why Novichok was used on UK soil.
In a statement in the Commons on Monday, Mrs May said it was now "clear" the Skripals had been poisoned with a military-grade nerve agent "developed" by Moscow.
She told MPs Russia's UK ambassador has been summoned to the Foreign Office over the incident on March 4 to be grilled over whether the Kremlin directly ordered the attack.
Amber Rudd announced on Tuesday that MI5 and the police are to look into allegations that a string of deaths on UK soil may be linked to Russia.
It follows reports claiming that US intelligence sources suspect as many as 14 people may have been assassinated on British soil by Russia's security services or mafia groups.
The examinations could be carried out despite inquests and investigations in each case - including the deaths of prominent Putin critic Boris Berezovsky and whistle-blower Alexander Perepilichnyy while out jogging - at the time finding no evidence of foul play.
Meanwhile, Boris Johnson said the UK has been encouraged by the "willingness" of countries around the world to "show support and solidarity" with Britain in the wake of "the first use of a nerve agent in Europe since the Second World War".
France and Germany are among those who have backed the Government and Mr Johnson said there was "a pattern of disruptive behaviour, increasingly disruptive behaviour, malign behaviour by Russia, the reckless use of chemical weapons, the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury".
Mr Johnson said that a range of "commensurate but robust" measures are being considered if it is proven Russia was behind the "brazen attempt to murder innocent people on UK soil".
The foreign secretary added that it is "overwhelmingly or highly likely that the Russian state was involved" in the poisoning.
Theresa May has warned that expelling diplomats and freezing assets, measures carried out by the UK in the wake of Alexander Litvinenko's death, might not be sufficient over the Skripal attack.
How has Russia responded?
The Foreign Secretary's comments came as his Russian counterpart, Sergey Lavrov, stated that his country is "not to blame" for the poisoning of Sergei and Yulia Skripal, adding that Russia is demanding access to samples of the nerve agent if the UK wants it to cooperate in the investigation.
However, it appears Russia is unlikely to meet Mrs May's deadline after the country's Foreign Ministry gave British Ambassador Laurie Bristow a protest note over the "baseless accusations" leveled against Russia.
The ministry dismissed the reaction of British authorities to the attack as "provocative" and said it suspects the poisoning is "another unscrupulous attempt of the British authorities to discredit Russia."
The ministry had previously described Mrs May's comments that it was "clear" the Skripals had been poisoned with a military-grade nerve agent "developed" by Moscow as a "circus show".
On Monday, while visiting a grain centre, Russian leader Vladimir Putin dismissed Kremlin links.
"We're dealing with agriculture here ... and you talk to me about some tragedies," he told the BBC.
"Get to the bottom of things there, then we'll discuss this
Earlier a Kremlin spokesperson said: "We have nothing to do with the story."
While on Tuesday, a Russian MP compared Britain to Hitler over the poisoning of "the useless fake agent" and branded the whole incident "fake news".
Speaking to the BBC, Vitaly Milonov said the Government had "created a fake story because they need an explanation to British people and British business why they are going to perform some anti-British steps in favour of United States policy against Russia.
"So, they had to make this fantasy. They had to kill, to try to kill, this poor pensioner, useless for us."
What is the latest from the police?
The investigation into the poisoning is likely to continue "for many weeks" and the prime focus is now on how the poison was administered, police have said in their latest update into the attack.
In a statement, counter-terrorism chief Neil Basu said that both Sergei and Yulia Skripal remain in critical conditions, and while Detective Sergeant Nick Bailey's condition is still "serious" he is "making good progress".
Mr Basu continued that of the 35 other people who sought medical attention following the incident, 34 had been dismissed by medical staff, while one continues to be monitored as an outpatient, but is not showing any signs of illness.
He continued that police would "leave no stone unturned" in their investigation and appealed for the public's help.
The former Met commander said no person of interest or suspect had been declared so far.
Mr Basu said police were keen to speak to anyone who was in Salisbury on the afternoon of Sunday, March 4, and for anyone who saw Mr Skripal's red BMW, with registration HD09 WAO being driven in the city between 1pm and 1.45pm on that day.
He added that residents would continue to see an increased police presence in Salisbury over the coming days and that more cordons may be imposed as the investigation continues.
He concluded by thanking "the public for their patience and understanding" as well as the "brave officers who responded to the incident and those who continue to work tirelessly around the clock" supported by members of the fire service and also the military.