Nato’s Secretary General has warned Moscow it has underestimated the West’s determination to oppose Russia, as he set out how the alliance has tripled its European reserve forces.

Jens Stoltenberg spoke after Nato’s top political council received a briefing from the British Government on the on-going investigation into the Salisbury nerve agent attack.

The incident which left ex-spy Sergei Skripal, his daughter and a British police officer in a serious condition has galvanised the West to confront Russia over the incident and what they called a “pattern” of unacceptable behaviour.

The Kremlin is expected to set out retaliatory measures after the UK announced it would expel 23 Russian diplomats earlier this week, freeze assets and seek new powers to deal with “hostile activity”.

Speaking to BBC Radio 4, Mr Stoltenberg said: “I’m absolutely certain that Russia has underestimated the resolve and the unity of Nato allies.”

On Thursday the leaders of France, Germany the US and UK took the unprecedented step of issuing a joint statement blaming Russia for the 4 March Salisbury attack involving a Russian-made Novichok nerve agent.

But they went further saying, the attack was only the latest element of a series of “unacceptable” incidents taking in Russia’s invasion of Crimea and destabilizing eastern Ukraine.

Donald Trump: It 'looks like' Russia responsible for nerve agent attack

Discussing defences in Eastern Europe, Mr Stoltenberg went on: “We have now reached the level of four battle groups we don’t have any immediate plans to increase the deployment.

“But what we are doing is increasing the ability to reinforce, to send in more troops quickly if needed.

“So we have tripled the size of what we call the Nato Response Force, we also established a high readiness brigade that can be deployed in a matter of days.”

Nato’s four multinational battlegroups, including a British-led one in Estonia, and others in Latvia, Lithuania and Poland, totalling 4,500 troops, became fully operational in 2017.

Three years earlier, the alliance set about increasing the size of the Nato Response Force to 40,000, with the high-readiness Spearhead Force at its core.

Mr Stoltenberg added: “It’s important Russia gets a clears signal that it has consequences, costs, to behave the way they behave.

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

“All Nato allies express a strong political support for the UK, the United Kingdom is not alone.”

Russia's Ambassador to the UK, Alexander Yakovenko, yesterday described Ms May's expulsion of diplomats as "absolutely unacceptable" and "a provocation" and urged her to "follow international law".