Theresa May said the diplomats, who have a week to leave, were identified as "undeclared intelligence officers".
The UK later told the UN Security Council that Russia had used "a weapon so horrific that it is banned in war" in a "peaceful" British city.
UK to expel 23 Russian diplomats
The UK will expel 23 Russian diplomats after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in Salisbury, the PM says.