Theresa May has condemned the "horrific" bombing in St Petersburg which killed 11 people on Monday.
The prime minister said she had written to Russian President Vladimir Putin to express her condolences for the "absolutely appalling attack".
"Our thoughts must be with the victims and their friends and families," she said.
"This shows the terrible terrorist threat we are all facing.
"I'm very clear, as the people of the UK were after the attack in London, that we will prevail, the terrorists will not win, our values will prevail."
The seven people were from Central Asian countries and investigators say they were recruiting for IS.
The bombing of a subway train in Russia has drawn condemnation from world leaders.
A blast has ripped through an underground train in Russia's second-largest city on Monday.