British authorities said they were poisoned with Novichok, a military-grade nerve agent developed in Russian Federation.
Skripal and his daughter Yulia have been critically ill since they were found unconscious on a bench in the southern English city of Salisbury on March 4.
The source of the nerve agent used - which Britain says is the Soviet-made Novichok - is unclear, as is the way it was administered.
Russia's Defence Ministry said he was an "intellectual impotent" and Lavrov said he probably lacked education.
On Wednesday, the White House said it "stands in solidarity" with "its closest ally" the United Kingdom and supported its decision to expel the Russian diplomats.
She also condemned what she said was Russia's contemptuous response to allegations that Moscow deployed a chemical weapon in Europe.
May spoke in the House of Commons after she chaired a National Security Council meeting to hear the latest evidence in the case.
Geopolitical tensions are mounting since the poisoning of ex-spy Sergei Skripal and his daughter in the English city of Salisbury earlier this month, in what Western powers see as the latest sign of increasingly aggressive Russian meddling overseas.
He backed the May government's actions but said going after Russian crony capitalism in Britain would have far greater impact than "tit-for-tat expulsions".
Haley called the crisis a "defining moment" for the U.N. and said the United States was solidly behind Britain.
Britain Blames Russia's President For Nerve Agent Attack
"We will not tolerate such a brazen attempt to murder innocent civilians on our soil", May added.
Mirzaynov was sacked after revealing the existence of Moscow's classified programme to produce Novichok and now lives in the United States. Peskov reiterated Russian denials of involvement in the attack that has left both Skripals in critical condition.
Boris Johnson, while visiting the Battle of Britain Bunker museum in Uxbridge with his Polish counterpart, told reporters the British government has "nothing against" the Russian people and that their problem is with the Kremlin.
"There has been a lot of tough talk over the years and nearly no action to protect our national security and integrity", he said.
In connection with the introduction by Britain of unfriendly steps towards Russian Federation, we plan to introduce reciprocal steps, without doubt. "We have talked tough by calling it names and expressing our dislike of the regime, but at the same time we have been very much open for business with Russian Federation, of any kind".
The official military newspaper Krasnaya Zvezda (Red Star) interviewed the former head of Russia's GRU military intelligence, Fyodor Ladygin, who denied his officers had been involved.
The apparent transatlantic willingness to square up to Moscow was bolstered by the announcement from Washington that it had placed sanctions on 19 Russians and five organisations, among them Russia's intelligence services, for meddling in the 2016 presidential election and a two-year campaign of cyber attacks on the American power network, including nuclear facilities.
Moscow is now preparing its response to Mrs May's expulsion of diplomats, with a tit-for-tat reaction expected to the decision to kick out 23 diplomats who she said were undeclared intelligence officers.More news: Arizona Stars Deandre Ayton and Allonzo Trier Are Leaving for the National Basketball Association
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