Kremlin's demand to Britain: Show proof of poisoning or apologize

March 19 (UPI) -- The Kremlin demanded Monday that British officials either show proof that Russia was behind the near-lethal poisoning this month of a former spy and his daughter, or apologize.

London officials, including Prime Minister Theresa May, have directly accused the Russian government of orchestrating the poisoning attack on former double agent Sergei Skripal and his adult daughter in a British shopping district March 4.

The pair were found slumped over on a bench in the Salisbury district and hospitalized. Investigators have said they were poisoned with a Russian-made nerve agent. They remain in critical condition.

"Sooner or later they will have to be responsible for these allegations: they will either have to provide some evidence or apologize," Kremlin Spokesman Dmitry Peskov said Monday, calling Britain's accusations "difficult to explain" and "groundless and slanderous."

Moscow's statement came on the same day the European Union condemned the poisoning.

Calling the incident "reckless and illegal," the EU echoed British, U.S., German and French statements in saying it's "highly likely" Russia was responsible.

"The European Union is shocked at the offensive use of any military-grade nerve agent, of a type developed by Russia, for the first time on European soil in over 70 years," the EU said in a statement. "The use of chemical weapons by anyone under any circumstances is completely unacceptable and constitutes a security threat to us all."

Russia has faced growing international criticism over their alleged role. British Foreign Secretary Boris Johnson said it's "overwhelmingly likely" that Russian President Vladimir Putin -- who was re-elected for a fourth term on Sunday -- ordered the poisoning himself.

Last week, May expelled 23 Russian diplomats over the attack -- the single largest expulsion in more than 30 years.

In response, the Kremlin expelled 23 British diplomats from Russia, closed the British Council and ended an agreement to reopen the British consulate in St. Petersburg.

Russian officials also suggested the toxin used in the attack, Novichok, may have come from a British research laboratory located not far from the scene of the attack in Salisbury.