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British Prime Minister Theresa May vowed to "dismantle the Russian espionage network in the UK” in response to the poisoning of a former double agent on Wednesday, announcing the expulsion of 23 Russian diplomats.

She said the Russians were "identified as undeclared intelligence officers" and gave them one week to leave Britain.

"This will be the single biggest expulsion for over 30 years and it reflects the fact that this is not the first time that the Russian state has acted against our country," she told MPs.

Britain expelled four Russian diplomats in 2007, in response to the radioactive poisoning of former agent Alexander Litvinenko.

After a record number of 105 Soviet diplomats being thrown out in 1971, there were further expulsions in 1985, 1989 and 1996.

May announced the move as part of a series of measures against Moscow on Wednesday in which she  for the first time, directly held Russia responsible for the poisoning.

"There is no alternative conclusion other than that the Russian State was culpable," May said.

May stressed the Kremlin had offered "no explanation" of how a nerve agent developed in Russia came to be used in the attack in Salisbury, southwest England, declaring that its response showed "complete disdain for the gravity of these events".

"They have treated the use of a military grade nerve agent in Europe with sarcasm, contempt and defiance," she said.

As part of the measures, May confirmed members of the British royal family as well as MPs will not be attending this year’s World Cup in Russia, although the move does not affect the England team's participation in the tournament.

May also vowed to freeze Russian state assets in the UK if evidence shows there is a threat to the UK or its residents.

"There is no place for these people - or their money - in our country," she warned.

May pledged to "urgently" develop new legislation to give law enforcement the ability to detain those "suspected of hostile state activity" trying to enter Britain. Such a power is currently only used against terrorism suspects.

Lawmakers' ability to impose sanctions on those responsible for rights violations will be strengthened, May said, as part of legislation named after anti-graft lawyer Sergei Magnitsky who died in Russian detention.

The government will also look into new counter-espionage laws "to clamp down on the full spectrum of hostile activities of foreign agents in our country", May said.