In their report to Mrs May, The Sun has learned that MI5 and MI6 chiefs will cite the very rare substance used on ex-spy Sergei Skripal and daughter Yulia as key evidence of the Kremlin’s involvement.
It is believed to have been developed in the SVR Russian foreign spy service’s notorious Yasenevo laboratory.
Mrs May will then summon an emergency meeting of her National Security Council at 11 am to decide on the scale of Britain’s retaliation.
The result of the finding could be more economic sanctions against Russia (which is still facing sanctions tied to the annexation of Crimea).
But it is feared that a strong reaction ahead of Russia’s presidential elections next Sunday may play into Putin’s hands.
It is suspected that the Russian ruler sanctioned the brazen nerve agent attack simply to goad Britain into a reaction that he can strike back against and look like a strongman standing up to the West to voters.
Former British Ambassador to Russia Sir Tony Brenton said yesterday: “The more Putin can point to Western hostility and aggression, the more he rallies the Russian people around him”.
Sir Tony added: “Russia is number one on a list of suspects that doesn’t include a number two”.
In a hint of action to come, the Chancellor said: “If there were to be an involvement of a foreign state, then obviously that would be very serious indeed and the government would respond appropriately.
Philip Hammond also told BBC1’s Andrew Marr Show that Britain will not be humiliated by the attack, that breaks every rule in the international book.
He added: “The vast resources that have been deployed and the high level assets that we have been able to use show that nobody is laughing at us.
“This is a very serious investigation. Let’s see where it leads us.”
Mrs May came under mounting pressure last night from campaigners and her own MPs to hit back at Russia.