Skripal, 66, and his daughter Yulia, 33, were found slumped unconscious on a bench outside a shopping center in the genteel southern English city of Salisbury on March 4.
The White House issued a statement Wednesday that said the USA "stands in solidarity with its closest ally, the United Kingdom" and shares in the assessment "that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support Britain's decision to expel Russian diplomats as a just response".
After the first known offensive use of such a weapon on European soil since World War Two, Britain has pinned the blame on Moscow and given 23 Russians who it said were spies working under diplomatic cover at the London embassy a week to leave.
The source of the nerve agent - which Britain says is Soviet-made Novichok - is unclear, as is the way it was administered.
US President Trump's spokeswoman accused Russian Federation of undermining the security of countries worldwide.
Before May's remarks to Parliament, the British Foreign Office said the U.K. delegation to the United Nations had requested an urgent meeting of the Security Council, "to update Council members on the investigation" into the nerve agent attack.
Russian foreign minister Sergei Lavrov said Moscow would co-operate in the case if it received a formal request for clarification from the United Kingdom under the Chemical Weapons Convention, which sets a 10-day time limit for a response.
In a speech on Thursday the Defence Secretary blasted Moscow for the "reckless" nerve gas attack in Salisbury and said Russian Federation should "go away and shut up". Thursday, Kremlin spokesman Dmitry Peskov said Russian President Vladimir Putin is mulling retaliatory measures against London.
Soon after Johnson's comments were reported, the Kremlin said accusations that President Putin was involved in the nerve agent attack were shocking, TASS news agency reported.
The joint statement represents a major boost for Mrs May and comes one day after she moved to expel 23 Russian diplomats and suspended high-level contact with Moscow in response to the Salisbury incident.
Russian spy Sergey Skripal and his daughter Yulia have been in intensive care after they were found unconscious on a bench in Salisbury almost two weeks ago. "If we don't take immediate action, they could be used here in NY", she said. It will also investigate the attack on Yulia Skripal, it said in a statement on its website.
The attack sparked a public clash between Russian Federation and the United Kingdom, with May threatening "economic war" after Moscow refused to answer her questions.
Britain said the Russians being expelled were undeclared intelligence officers.
Britain said the assistance in that case was not enough, and in 2016, a judge-led inquiry concluded that Putin had probably approved Litvinenko's murder, something Moscow denies.
Announcing the measure, Mr Williamson said Russian Federation "should go away - it should shut up".
An invitation for Russian Foreign Minister Sergey Lavrov to visit Britain has been cancelled, and British ministers and royals won't attend the soccer World Cup in Russia this summer.