Boris Johnson has said the the Kremlin “must give answers” after an international watchdog confirmed that Sergei Skripal and his daughter Yulia were poisoned with a "high purity" strain of Novichok nerve agent in Salisbury.
The Organisation for the Prohibition of Chemical Weapons said that it had been able to “confirm” the findings of British scientists about the nerve agent.
It represents a significant boost to Theresa May, who has said that Russia was directly responsible for the attack. Mr Johnson said that only Russia has the “means, motive and record” to have carried out the attack.
He said: “We will now work tirelessly with our partners to help stamp out the grotesque use of weapons is this kind. The Kremlin must give answers. We must, as a world community, stand up for the rules based order which keeps us all safe. The use of weapons of this kind can never be justified, and must be ended.”
The OPCW conducted tests on blood samples from the Skripals and also an analysis on samples of the agent found in Salisbury.
The team also took samples from Detective Sergeant Nicholas Bailey, who was poisoned after coming into contact with the agent while assisting the Skripals.
The report states: “The results of analysis of biomedical samples conducted by OPCW designated laboratories demonstrate the exposure of the three hospitalised individuals to this toxic chemical.
“The results of the analysis of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people."
The team notes that the toxic chemical was of "high purity".
British scientists have suggested that only a foreign state would have been capable of producing the nerve agent.
Video: Watchdog Confirms Findings on Former Double-Agent Attack
It comes as Ms Skripal revealed she has rejected assistance from the Russian embassy, adding: "I want to stress that no one speaks for me, or for my father, but ourselves."
The 33-year-old said she has found herself in a "totally different life" as she continues to recover from the attack.
The Russian embassy said that it doubted the authenticity of the statement and suggested it had been crafted to support Britain's version of events.
It is believed that British authorities immediately spirited Ms Skripal away to a secure location when she was discharged from hospital earlier this week.
The Russian embassy reacted angrily, suggesting in a series of tweets that the Russian national had been taken against her will.
The Kremlin has repeatedly attempted to challenge Britain's claims that it was responsible for the poisoning of the Skripals.
War of words as diplomatic battle continues to rage
The Russian embassy is perturbed by a refusal from UK authorities to grant Ms Skripal's cousin Viktoria a visa to visit her family.
It escalated the war of words on Tuesday, saying in a series of statements posted on social media: "Secret resettlement of Mr and Ms Skripal, barred from any contact with their family will be seen as an abduction or at least as their forced isolation."
But Ms Skripal said in a statement: "I have specially trained officers available to me, who are helping to take care of me and to explain the investigative processes that are being undertaken.
"I have access to friends and family, and I have been made aware of my specific contacts at the Russian embassy who have kindly offered me their assistance in any way they can.
"At the moment I do not wish to avail myself of their services, but, if I change my mind I know how to contact them.
"Most importantly, I am safe and feeling better as time goes by, but I am not yet strong enough to give a full interview to the media, as I one day hope to do.
"Until that time, I want to stress that no one speaks for me, or for my father, but ourselves.
"I thank my cousin Viktoria for her concern for us, but ask that she does not visit me or try to contact me for the time being. Her opinions and assertions are not mine and they are not my father's."
The Russian embassy later said that the statement "raises new questions rather than gives answers" as it was unable to verify it.
"The text has been composed in a special way so as to support official statements made by British authorities and at the same time to exclude every possibility of Yulia's contacts with the outer world - consuls, journalists and even relatives," the embassy said in a statement.
"The document only strengthens suspicions that we are dealing with a forcible isolation of the Russian citizen."
Pair poisoned by Novichok, experts conclude
Theresa May has said that UK military experts at Porton Down found that they were poisoned by Novichok, a nerve agent developed by Russia, and that the only plausible explanation was that the Russian state was responsible.
Despite the denials from Moscow, Britain has received strong diplomatic backing from western allies for its stance, with more than 20 countries expelling in excess of 150 Russian diplomats in protest at the Russian actions.
Last week, a Russian call for a new joint investigation was voted down by 15 votes to six at a meeting of the OPCW executive council in The Hague.
Ex-double agent jailed for selling secrets to MI6
Former double agent Mr Skripal was jailed in Russia for selling secrets to MI6 but was released as part of a spy swap deal in 2010 and settled in the UK.
It is hoped he will soon be fit for release from hospital, despite grave fears that the exposure to military-grade Novichok on March 4 would prove fatal.
Ms Skripal said she was treated with "such kindness" at Salisbury District Hospital, and is missing the staff there.
"I have left my father in their care, and he is still seriously ill. I too am still suffering with the effects of the nerve agent used against us.
"I find myself in a totally different life than the ordinary one I left just over a month ago, and I am seeking to come to terms with my prospects, whilst also recovering from this attack on me," she said.