Theresa May has used her party conference speech to warn Russia that the UK “will never tolerate a threat to the life of British citizens”, as the crisis around the Salisbury nerve attack deepens.

The Prime Minister said Moscow was in “flagrant breach” of international law over the incident which saw a Russian-made nerve agent release in the city.

She said: “Many Russians have made this country their home. And those who abide by our laws and make a contribution to our society will always be welcome.

“But we will never tolerate a threat to the life of British citizens and others on British soil from the Russian Government.”

But it comes after the Kremlin announced it would retaliate to Britain’s expulsion of 23 diplomats earlier this week, by also expelling the same number of UK officials from Moscow.

Saying that she had anticipated the move, the Prime Minister went on: “The rule of law. Freedom of speech. The toleration of dissenting and minority views. A free press. Fair and democratic elections. A thriving civil society. These are the foundation stones of human freedom.

“They don’t come about by accident, and they are certainly not the default setting for any society.

“They take years of patient work to build up, they face constant threats, and they must always be defended. They are the values which unite us as a country.”

The overt defence of national values comes after a week in which Ms May criticised Jeremy Corbyn for failing to blame Russia for the attack, something which also divided the Labour frontbench.

The Russian Foreign Ministry announced said on Saturday morning that it would expel 23 British diplomats in a like-for-like retaliation to British measures, saying the diplomatic representatives of the British Embassy in Moscow should leave Russia within a week.

It came four days after Ms May announced 23 Russian diplomats would be expelled from Britain after Russia missed a deadline to provide an explanation for the poisoning of Sergei Skripal and his daughter Yulia.

The ex-spy, his daughter and a British policeman are still in serious conditions in hospital following the 4 March attack which saw a military grade Novichok nerve agent released in the Salisbury.

Responding to Russia's announcement, the UK Foreign Office said in a statement it had “anticipated a response of this kind” and the National Security Council would meet early next week “to consider next steps”.

It insisted there was “no alternative conclusion other than that the Russian State was culpable” but said it was not in the UK's national interest “to break off all dialogue between our countries”.

“The onus remains on the Russian state to account for their actions and to comply with their international obligations,” the statement concluded.

Vladimir Putin's Photo ops – Russia's Man of Steel? Show all 20 1 /20 Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin is pictured with a horse during his vacation outside the town of Kyzyl in Southern Siberia on August 3, 2009. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin aims at a whale with an arbalest to take a piece of its skin for analysis on the Olga Bay, some 240 kilometres north-east of Nakhodka on August 25, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin plunges into the icy waters of lake Seliger during the celebration of the Epiphany holiday in Russia's Tver region AFP/Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin (top) takes part in a judo training session at the "Moscow" sports complex in St. Petersburg, on December 22, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin works out at a gym at the Bocharov Ruchei state residence in Sochi on August 30, 2015. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin fishes in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin (L) and the leader of the Night Wolves biker group, Alexander Zaldostanov (R), also known as the Surgeon, ride motorcycles on August 29, 2011 at a bikers' festival in the Black Sea port of Novorossiysk, Russia. Putin described leather-clad bikers as brothers and boasted of the "indivisible Russian nation" after roaring into a biking rally on a Harley Davidson. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin wears glasses as he visits the Technology Park of the Novosibirsk Academic Town in Novosibirsk on February 17, 2012. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin rides a horse during his vacation outside the town of Kyzyl in Southern Siberia on August 3, 2009. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin holds a pistol during his visit to a newly-built headquarters of the Russian General Staff's Main Intelligence Department (GRU) in Moscow, 08 November 2006. ?Some countries are seeking to untie their hands in order to take weapons to outer space, including nuclear weapons,? Putin said at the Chief Military Intelligence Department on Wednesday. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin sits inside a T-90AM tank during a visit to an arms exhibition in the Urals town of Nizhny Tagil on September 9, 2011 Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin wears a helmet and the uniform of the Renault Formula One team before driving a F1 race car on a special track in Leningrad region outside St. Petersburg on November 7, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin guides a boat during his vacation in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin hunts fish underwater in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin is seen at the Russian boxing team training club after casting his vote for the Russian Presidential election, 14 March 2004 in Moscow. Putin coasted to a landslide victory with 69.0 percent of the vote in Sunday's election, according to the first exit poll aired on Russian television moments after voting ended across the country's 11 time zones. AFP/Getty Images Vladimir Putin's Photo ops – Russia's Man of Steel? Russian President Vladimir Putin poses for a picture inside the Tupolev-160 strategic bomber jet at the Moscow's Chkalovsky military airport, 16 August 2005. President Vladimir Putin took off from Moscow for a supersonic flight in a cruise-missile carrying Tupolev-160 bomber jet, the latest in the Russian leader's action-packed public appearances. After a health check, Putin donned a flight suit and took the commander's position in the strategic bomber, which was piloted by Major General Anatoly Zhikharev, with a colonel and a lieutenant colonel in charge of navigation, Russian media reported. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? A picture released on March 6, 2010 shows Vladimir Putin look through binoculars in the Karatash area, near the town of Abakan, during his working trip to Khakassia, on February 25, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin measuring a polar bear on the island Alexandra Land, part of the Franz Josef Land archipalego in the Arctic Ocean. Putin, better known in the West for his tough-guy image, expressed concern for the fate of Arctic polar bears threatened by climate change. "The polar bear is under threat. Their population is currently only 25,000 individuals," Putin was quoted by Russian news agencies as saying Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin carries a hunting rifle during his trip in Ubsunur Hollow Biosphere Reserve in Tuva Republic in this undated picture released on October 30, 2010 by RIA Novosti news agency. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Russian President Vladimir Putin pilots a motorized hang glider while flying with cranes as he takes part in a scientific experiment as part of the "Flight of Hope", which aims to preserve a rare species of - cranes on September 5, 2012. At the helm of a motorized hang glider that the birds have taken as their leader, Putin made three flights - the first to get familiar with the process, and two others with the birds. AFP/Getty

A Russian response to the British measures had been expected for several days and when it came, it went further than expected.

Apart from the expected tit-for-tat expulsions, the Ministry of Foreign Affairs announced it is stopping all British Council activities “due to legal irregularities” and revoking its agreement for Britain to operate a consulate-general in St Petersburg.