The diplomats will have to leave the UK within a week, May said. She also announced her decision to suspend all top level ties with the Russian Federation.
'Under the Vienna Convention the United kingdom will now expel 23 Russian diplomats who have been identified as undeclared intelligence officers - they have just one week to leave.
"They have treated the use of this poison with sarcasm and disdain...There is no other conclusion other than that the Russian state was responsible for the attempted assassination of Sergei Skripal and his daughter," Theresa May said. This will be the single biggest expulsion in over 30 years and it reflects the fact this is not the first time that the Russian state has acted against our country.
"Through these expulsions we will fundamentally degrade Russian intelligence capability in the UK for years to come. And if they seek to rebuild it we will prevent them from doing so," she added.
"We will also table a government amendment to the Sanctions Bill to strengthen our powers to impose sanctions in response to the violation of human rights," she said.
"In doing so we will play our part in the international effort to punish those responsible for the sorts of abuses suffered by Sergei Magnitsky."
The British government has also called for an urgent meeting of the UN Security Council.
Russian Ambassador to the UK Alexander Yakovenko has already warned London of responsive measures. According to the Russian ambassador, Moscow considers Britain's actions a provocation. British Ambassador to Russia Laurie Bristow was summoned to the Russian Foreign Ministry for talks.
The relations between Russia and the UK hit another low after former Russian spy Sergei Skripal and his daughter Yulia were found unconscious on a bench in Salisbury. The British government considers Russia involved in the accident, but Russia strongly rejects these accusations.