The UK has rejected a complaint from Russia about the search of one of its planes at London's Heathrow Airport as a dispute over the poisoning of a former Russian spy intensifies.
The British government described the search for the Aeroflot plane on Friday as a "routine" to Russia, suggesting it was "illegal".
The poison dispute has expelled dozens of diplomats on both sides.
Moscow wants to reduce the representation of Britain in Russia by another 27 diplomats.
In response to the announcement by the United Kingdom of 23 Russian expulsions on 1
Ex-spy Sergei Skripal, 66, and daughter Yulia, 33, were poisoned in Salisbury on March 4, prompting British investigators to conclude that it was a Russian neonate named Novichok. The Russian denies the participation.
He is critically ill but stable while now being told that she is aware and speaks.
The Foreign and Commonwealth Offices have since declared that it will consider a request for Russian consular access to Yulia, a Russian citizen.
What happened on the plane?
The Aeroflot SU2582 landed at 15:40 local time on Friday at Heathrow.
A man identified by Russian television as Pilot Vitaly Mitrofanov said he had been locked into his cockpit by a team with a beagle.
The border officers controlled the plane for up to three minutes before leaving with a "thank you," he added.
The pilot claims that they gave "no grounds" for the inspection, and he was not allowed to "watch their actions" or accompany them in their work.
"If there is no explanation, the Russian side will consider the actions against our aircraft illegal and reserves the right to take similar action against British airlines," the Russian Ministry of Transport said in a statement.
Russian Foreign Ministry spokeswoman Maria Zakharova called the search a "provocation" so that the British government could "save its face in every possible way".
How did Britain react?
British Security Secretary Ben Wallace said in a statement "routine for Border Force to search planes to protect the United Kingdom from organized crime and from those who try to bring harmful substances such as drugs or firearms into the country ".
He added, "As soon as these checks were made, the plane was allowed to continue its journey."
Former UK Bureau chief Rob Whiteman said intelligence was a factor.
"This incident, when the Aeroflot flight was searched at Heathrow, would have been a routine security search by the security services to respond to intelligence information," he told the BBC.
"These searches are routine – they do not happen every day, but they happen fairly regularly."
What is the background of the recent dispute?
The poisoning series has been hunting for almost a month.
The United Kingdom is relentless Russia is behind the poisoning of scripts – Russia denies it.
Moscow is stunned by the strength of support for Britain by mainly Western countries, which is the expulsion of 150 Russian diplomats over the course of three days earlier this week.
Moscow responded in the same way and then added the number of British diplomats who were to leave Russia.
On Friday, British Ambassador Laurie Bristow was told that Britain had one month to reduce its diplomatic mission in Russia to the same size as the Russian mission in the UK.
A spokeswoman for the British Foreign Ministry said: "The response of Russia is regrettable, but we expected a reaction in view of Russia's past behavior."
"But that does not change the fact: the attempted murder of two people on British soil for which there is no alternative conclusion, except that the Russian state is guilty.
UN Secretary-General Antonio Guterres issued a warning on Thursday "of a situation that is very similar to what we experienced during the Cold War."
Russia re-elects Britain
] By Paul Adams, BBC News, Moscow
British diplomats left Moscow a week ago, but Ambassador Laurie Bristow was recalled to the Foreign Ministry for additional punishment.  Beyond that is The Russian side has not really explained why
But it is clear that Russia sees Britain as the leader of an international conspiracy that has led to the largest mass displacement of Russian diplomats in history.
Britain's "provocative and unjustified actions," the ministry said "inspired the unfounded expulsion of Russian diplomats."
It is a devious compliment to Prime Minister Theresa May, who has successfully circled a broader international coalition than anybody would have thought possible a month ago.
Russia has opposed the British government for its efforts to internationalize the so-called "Skripal affair". The solidarity expressed by so many countries has been dashed by financial and political pressure along with the US.
How does Russia view the expulsions?
Russian Foreign Minister Sergei Lavrov has made "hard pressure" from the United States and Britain under the pretext of the so-called "Skripal case" for the decision of dozens of countries to expel Russian diplomats.
He repeated the Russian Demands for consular access to Yulia Skripal – a Russian citizen – The UK Foreign Office says it judges it "in line with our obligations under international and national law, including the rights and wishes of Julia Skripal."
Russia, Lavrov also sought a meeting with leaders of the Organization for the Prohibition of Chemical Weapons (OPCW) to "find the truth."