The US state department will impose sanctions on Russia after it determined Moscow used a nerve agent to attack the Skripals.
The US expelled a number of Russian diplomats as part of an international response to the attack, when the former double agent Sergei Skripal and his daughter were taken ill in March.
State Department spokeswoman Heather Nauert said: “Following the use of a ‘Novichok’ nerve agent in an attempt to assassinate UK citizen Sergei Skripal and his daughter Yulia Skripal, the United States, on 6 August 2018, determined under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991 (CBW Act) that the Government of the Russian Federation has used chemical or biological weapons in violation of international law or has used lethal chemical or biological weapons against its own nationals.”
She said sanctions would take effect on or around 22 August, after a 15-day Congressional notification period.
NBC reports a senior official as saying the initial sanctions will be a ban on granting licenses to export sensitive national security goods to Russia.
Mr Skripal and his daughter Yulia were in intensive care for several weeks after being poisoned with the chemical substance at his home in Salisbury in March.
They were found in a serious condition on a bench in The Maltings shopping centre, and the discovery of the substance sparked a city-wide investigation with police having to cordon off large areas, including where they had eaten dinner that evening.
Three months later, Dawn Sturgess and her partner Charlie Rowley fell ill after they came into contact with the substance, which was found in a sealed perfume bottle.
The UK government has accused Russia of being behind the attack, but the Kremlin has continuously denied involvement.
Republican strategist Matt Mackowiak told Sky News: “The state dept announced they would undertake their own process to come to their own conclusions when this happened.
“This backs up what other intelligence services and the British services have found. It shows the special relationship between the UK and the US.
“President Trump believes there is a way to work with Russia where they have common issues, but their behaviour gets worse, and the US has to be part of the international response.
“Most of the allied nations are relieved to see this step happening today.”
He said the sanctions were “significant”, even if their response had been slower than others.
Mr Mackowiak also said there could be a push from Congress for even greater sanctions amid the investigation into Russia’s alleged meddling in the 2016 election.
The news of sanctions comes hours after Senator Rand Paul said he delivered a letter from Mr Trump to Russian president Vladimir Putin which proposed “further engagement in various areas including countering terrorism, enhancing legislative dialogue and resuming cultural exchanges”.
Mr Trump has invited Mr Putin to the White House.
Reports earlier this week that the UK was preparing to request the extradition of two Russians it suspected of being behind the attack were found to be less advanced than initially suggested, Sky sources said.
Russia’s embassy in London said they had not received any extradition requests over allegations any of its citizens were involved in the novichok poisonings.
The Skripals were targeted in the March attack, but the second pair to be affected came across the poison in a perfume bottle they found in a bin in Amesbury, eight miles away, in July.
Ms Sturgess died, having sprayed the substance over herself, but Mr Rowley survived.