The embassy said British Border Forces and Customs officers searched an Aeroflot flight from Moscow Friday in violation of international rules, AP reported.
In a tweet, the embassy called night called the alleged search "another blatant provocation by the British authorities" over the poisoning of a Russian ex-spy and his daughter in England.
The British government did not immediately comment on the embassy's complaint.
Russia says it has sent a diplomatic note demanding an explanation of the search.
Both sides have expelled diplomats as Britain blames Russia for the nerve agent attack on Sergei Skripal and his daughter.
Sergei Skripal remains in critical condition, while officials say Yulia Skripal is improving.