The measures include the expulsion of 23 Russian diplomats, the single biggest expulsion in over 30 years. May said the U.K. would also not send an official government representative to the soccer World Cup in Russia this year.

The U.K. "doesn't want to break off all dialogue but this relationship cannot be the same," May said in Parliament Wednesday, but announced that the U.K. is "suspending all high-level contact between the U.K. and Russia."

We have disagreements with the people of Russia, May said, and "it is tragic that President (Vladimir) Putin has chosen to act in this way."

She added that the U.K. does not stand alone in "confronting Russian aggression," saying that she had spoken with the leaders of the U.S., France and Germany.

The penalties mark the most serious deterioration in U.K.- Russia relations in recent years.

They come after Sergei Skripal, a former Russian spy turned double agent for the U.K., and his daughter Yulia, were subject to a nerve agent attack in early March for which they remain in a critical condition in hospital.

Earlier this week, May said it was "highly likely" that Russia was behind the attack that saw a Russian-developed nerve agent, part of the "Novichok" series of deadly chemical weapons, used against the pair.

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