The foreign secretary has hit out at Russia's 'reckless use of chemical weapons' as he thanked international leaders for their support in the wake of the chemical attack.
Boris Johnson said Britain had been encouraged by the "willingness of friends" to show support and solidarity following the poisoning of a Russian double agent.
US Secretary of State Rex Tillerson and the European Union are among those to have said they would stand by Britain after the attack.
Mr Johnson said he had talked to President Macron of France and with his German counterpart Sigmar Gabriel but he particularly paid tribute to the comments of his US opposite number Rex Tillerson.
He said: "Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour, increasingly disruptive behaviour, malign behaviour by Russia, the reckless use of chemical weapons, the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury."
Mr Tillerson's comments are particularly significant in the light of the White House's reluctance to condemn the Kremlin in recent days.
Former foreign secretary David Miliband this morning urged the PM to seek international support, telling BBC Radio 4's Today programme: "The biggest thing she has to do in the next two days is to rally her allies.
"It is very significant and very worrying, frankly, that the White House has not felt able to point the finger at Russia in the last seven or eight days.
"And, I think that rallying the European allies, and, if possible, significant strands of American opinion, is absolutely key."
Mr Johnson today: "What we're doing today is giving Russia until midnight tonight to explain how it came to be that novichok was used on the streets of Wiltshire.
"If they can come up with a convincing explanation then obviously we will want to see full disclosure of that to the Organisation for the Prohibition of Chemical Weapons in the Hague.
"If not, then clearly we will want to be announcing the UK response and that will come tomorrow.
"In the meantime what we've been doing is talking to friends and partners, explaining what we see as the high likelihood of Russian state agency.
"And I've been very encouraged so far by the strength of the support that we are getting.
Theresa May has given Russia a midnight deadline to explain how former spy Sergei Skripal, who passed secrets to British intelligence, and his daughter Yulia were poisoned with a nerve agent developed by the Soviet Union.