An Aeroflot Airbus A319-111 was seen at Sheremetyevo International Airport, Moscow on Dec. 1, 2014. (Web photo/Anna Zvereva)

LONDON, April 1 (Xinhua) -- Tensions continue to run high between Britain and Russia, with Moscow questioning the legitimacy an alleged search of a Russian passenger plane by British officials at London's airport on Friday, amid the aftermath of Salisbury spy poisoning incident.

According to Russia's Aeroflot airline, British authorities searched the SU2583 jet at London's Heathrow Airport on Friday without any explanation, in violation of the international practices.

The Russian Embassy to Britain termed the event as "extraordinary" and called it a "blatant provocation" by the British authorities in a statement released Friday.

It said it is "prohibited by the rules in force" for British officials to search the aircraft without the crew present, and the captain was allowed to take part in the search after "long negotiations."

Photo taken on March 14, 2018 shows a general view of the exterior of the Russian Embassy in London, Britain. (Xinhua/Stephen Chung)

The embassy also said the British officers refused to provide any written document that would specify the reasons for their actions, their legal foundation, and their outcome.

The Russian embassy said it has sent a diplomatic note demanding the British side provide an explanation, saying it reserved the right to act similarly against British airlines in Russia.

British Security Minister Ben Wallace said in a statement that it was "routine for Border Force to search aircraft to protect the UK from organized crime and from those who attempt to bring harmful substances like drugs or firearms into the country."

The Russia embassy also suggested in its statement that the plane search was in connection with the diplomatic crisis which has erupted following the poisoning incident in England.

Relations between Russia and the West have been strained after former Russian double agent Sergei Skripal and his daughter Yulia were found unconscious on a bench outside a shopping center in Britain's Salisbury on March 4.

Moscow denies involvement in the incident.

Russian Ambassador to the United Nations Vassily Nebenzia (Front) addresses a Security Council emergency meeting regarding accusations of the use of a nerve agent in the United Kingdom at the United Nations headquarters in New York, on March 14, 2018. (Xinhua/Li Muzi)

On Saturday, the Russian Foreign Ministry said Britain must cut over 50 more of its diplomatic and technical staff in Russia, after it summoned the British ambassador and told him on Friday that London has one month to downsize its diplomats stationed in Russia to the same level as Russia has in Britain.

Russia has already ejected 23 British diplomats and informed ambassadors of 23 countries of its decision to expel employees of their diplomatic missions in Russia in a tit-for-tat response to the West.

More than two dozen countries announced the expulsion of around 150 Russian diplomats, in a sign of support to Britain, which has expelled 23 of them.

(Xinhua reporter Liang Xizhi in London has contributed to the story)