Russia claims British authorities searched one of their planes at a London airport in what they called a "blatant provocation".
London's Metropolitan Police denied the force had inspected an Airbus plane that had landed at Heathrow Airport having departed from Moscow.
It said: "We are aware of a story circulating on social media. Please be advised that Metropolitan Police are not conducting a search of an Airbus inbound from Moscow at Heathrow."
The Russian embassy in the UK said in a statement: "Today, we have witnessed another blatant provocation by the British authorities.
"Border Force and Customs officers have searched the aircraft that was conducting the Aeroflot flights 2582 / 2583, Moscow - London - Moscow.
"This kind of event is extraordinary."
They also alleged British officials had tried to search the plane in the absence of the crew.
"After an embassy officer arrived at the airport, long negotiations were conducted that allowed to ensure the right of the captain to take part in the search," they added.
"After the search was over, the British officers refused to provide any written document that would specify the reasons for their actions, their legal foundation and their outcome.
"Our attempts to get in touch with the Foreign Office in order to clarify the reasons of the inappropriate behaviour of British officials did not bring fruit. The embassy has sent a diplomatic note demanding the British side to provide explanations of the incident.
"Of course, we will carefully analyse what has happened. At this moment, we have no other explanation but that the incident at Heathrow is in one way or another connected with the hostile policy that the UK government is conducting with regard to Russia."
The news comes as Russia continues to expel foreign diplomats in a tit-for-tat response to countries across the world sending home Russian diplomats in solidarity with Britain following the nerve agent attack in Salisbury.
One Irish diplomat is to be expelled from Russia, in a tit-for-tat response after Ireland confirmed one Russian diplomat would be expelled from here.