US President Donald Trump and British Prime Minister Theresa May agreed last night that Russia must provide "unambiguous answers" after London gave Moscow until midnight to explain how a Soviet-era nerve weapon was used against a former Russian double agent.

Mrs May, who said on Monday it was "highly likely" that Russia was behind the poisoning of Sergei Skripal and his daughter, also won support from Britain's main European allies and the European Union, which denounced the attack as "shocking" and offered help to track down those responsible.

Jens Stoltenberg, secretary-general of the US-led Nato alliance, said the attack was "horrendous".

Russia, however, signalled there was little likelihood that it would respond adequately to London's call for a credible explanation by today.

Denying it had played any part in the attack that left the 66-year-old Skripal and his 33-year-old daughter Yulia fighting for their lives, Russia said it would ignore the ultimatum until London handed over samples of the nerve agent used and complied with international obligations for joint investigations of such incidents.

"Any threats to take 'sanctions' against Russia will not be left without a response," the Russian foreign ministry said in a statement. "The British side should understand that."

Russia holds a presidential election on Sunday in which Vladimir Putin, himself a former KGB spy, is expected to coast to a fourth term in the Kremlin.

Skripal, a former officer with Russian military intelligence, betrayed dozens of Russian agents to British intelligence before being arrested in Moscow and jailed in 2006.

He was released under a spy swap deal in 2010 and took refuge in Britain, where he had been living quietly in the cathedral city of Salisbury until he and his daughter were found unconscious on a public bench there on March 4.

A British policeman who was also affected by the nerve agent is now conscious in a serious but stable condition.

Mrs May said on Monday Britain had identified the substance as belonging to the lethal Novichok group of nerve agents developed by the Soviet military in the 1970s and 1980s.

She and her ministers say Britain will take further "robust" punitive action against Russian interests - beyond sanctions already in place - if Mr Putin does not come up with a credible explanation of events.

Mr Trump's endorsement of Britain's tough line, made during a phone call with Mrs May, came on the heels of messages of solidarity from France's President Emmanuel Macron and German Chancellor Angela Merkel.

They gave the British prime minister additional hope of marshalling Western backing for her government as it heads towards a showdown with Putin.

"President Trump agreed with Prime Minister May that the Government of the Russian Federation must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom," the White House said in a statement about the two leaders' phone call.

"The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms," it said.

Mrs May's spokesman said Mr Trump had said the United States was "with the UK all the way".

It remains to be seen how much of a rupture in relations with Russia Mrs May's government is prepared to envisage.

While trade figures show Russia accounts for less than 1pc of British imports, London is of major importance for Russian companies seeking to raise capital and since the 1991 collapse of the Soviet Union has become the Western capital of choice for many Russian business leaders.

Britain could call on allies for a coordinated Western response, freeze the assets of Russian business leaders and officials, expel diplomats or launch targeted cyber attacks. It may also cut back participation in the soccer World Cup in June and July.

Russia Today, a round-the-clock news network that is funded by Mr Putin's government, could also lose its licence to broadcast in Britain.

Irish Independent