BEIJING, March 16 (Xinhua) -- Britain announced Wednesday that 23 Russian diplomats will be expelled from Britain due to Russia's failure to explain the poisoning of a former Russian spy.

The Russian ambassador to the United Nations also made an announcement Wednesday, saying that Britain's allegations were completely unacceptable and threatened reprisals against Britain if measures are taken by the British government.

Expelling diplomats is nothing new in the history of these two countries when one looks back at the past 200 years.

In 2006, ex-Russian agent Alexander Litvinenko was poisoned in London with the radioactive substance polonium-210, and the British government attributed his death to Russia.

Moscow responded with an investigation into British spies that posed as diplomats to Russia, and closed the branch of the British Council of Culture in Russia.

In September 1985, Moscow and London engaged in a furious six-day exchange of spy expulsions. Thirty-one Soviet citizens were expelled from Britain and 25 Britons were forced to leave Moscow.

This was followed by the defection of Col. Oleg Antonovich Gordievsky of the Soviet Committee for State Security (more commonly known as the KGB), who was a double agent. As the KGB's London station chief, he provided the British government with a name list of Russian spies there.

In 1971, Oleg Adolfovich Lyalin, who was working for the KGB and posted as an official with the Soviet Trade Delegation, was arrested in London by the police for drunk driving. Under the influence of alcohol, he leaked his relationship with the KGB.

His defection, however, led to a record number of 105 Soviet diplomats and officials being expelled by Britain after Moscow refused to clarify the activities of 440 of its citizens in Britain. Moscow in turn expelled 27 Britons.

In 1927, the British government announced it would sever diplomatic relations with Soviet Russia for its interference in China. However, the two sides had just established diplomatic relations in 1924.

As early as in 1800, British Ambassador to Russia Charles Whitworth was expelled from St. Petersburg for plotting to overthrow Tsar Paul I.