Russia yesterday said that Britain had to reduce its diplomatic staff by more than 50 more people as a crisis in ties between Moscow and the West escalated over the nerve agent attack on a former spy.

The new measures came after 23 British diplomats left Russia earlier this month and are seen as Moscow's punishment after Britain's allies expelled Russian diplomats over the March 4 poisoning of former double agent Sergei Skripal and his daughter Yulia in an English city.

"Russia suggested parity. The British side has more than 50 more people," foreign ministry spokeswoman Maria Zakharova told AFP.

On Friday, Moscow summoned British ambassador Laurie Bristow, giving London a month to cut the number of diplomatic staff in Russia to the same number Russia has in Britain.

He was summoned along with the heads of diplomatic missions from 23 other countries who were told that some of their diplomats had to leave, in the biggest wave of tit-for-tat expulsions in recent memory.

More than 150 Russian diplomats have been ordered out of the US, EU members, Nato countries and other nations.

France, Germany, Canada and Poland each said that Russia was expelling four of their diplomats. Other countries including Australia, Ukraine, the Netherlands, Sweden, the Czech Republic, Finland, Lithuania and Norway were also told to pull their envoys.

In the United States, 60 Russian diplomats expelled by Washington prepared to leave the country yesterday. In total, 171 people -- diplomats which Washington alleges are "spies" and their families -- were set to leave the United States, Moscow's envoy Anatoly Antonov told Russian reporters in Washington.

Britain has said it is "highly likely" that Russia was responsible for the Skripal attack using the Novichok nerve agent developed in the Soviet Union, but Russia has angrily denied any involvement.

Britain has suspended high-level diplomatic contact with Moscow and said it would not be sending any members of its royal family to the 2018 football World Cup hosted by Russia.