She promised "extreme measures", but some say they're anything but.
Theresa May is expelling 23 Russian diplomats after Russia rejected a deadline to explain the Salisbury nerve agent attack.
The British Prime Minister's also considering other actions, including possible boycotts.
But UK correspondent Rod Liddle says none of the measures will change Russia's position.
"I don't know what good can come of it. She's also said they're going to try and dismantle the Russian espionage network, I'd rather of hoped they were doing that already."
However, one expert on UK-Russian relations says Britain's expulsion of diplomats is a good first step.
British investigative journalist Edward Lucas has spent decades researching and reporting on Russia.
He told Mike Hosking the expulsion is mostly symbolic, but its significance shouldn't be underestimated.
"This is the biggest spy bust in Britain since the 1970s, so that's quite a big deal. We normally chuck out two, three, four - we're chucking out 23 this time."
Lucas says Britain's response to the Kremlin now needs to be backed up with broad-ranging expulsions and serious sanctions.
"One is where are the Allies? Because a synchronised expulsion is much more important than just one country doing it, and secondly: what happens next? Are we actually going to start making compromises?"
LISTEN ABOVE AS EDWARD LUCAS AND ROD LIDDLE SPEAK WITH MIKE HOSKING