Tensions over the separatist crisis in Ukraine have chilled diplomatic relations between London and Moscow, and earlier this week Foreign Secretary Philip Hammond said Russia could pose the greatest threat to British national security.
"We will be considering our representation (at the Moscow ceremony) in light of our ongoing discussion with Russia, our concerns about their activities, so I don't think we have plans for the prime minister to attend," the spokeswoman told reporters.
The European Union and United States accuse Russia of arming separatists in eastern Ukraine and reinforcing their ranks with troops. Moscow has repeatedly denied this and accuses the United States of pushing the pro-Western government in Kiev to war.
On Wednesday, German Chancellor Angela Merkel said the developments in Ukraine meant it would be impossible for her to take part in the World War Two ceremony.