Donald Trump has offered his strongest support yet for Theresa May, saying that the US shares the UK’s assessment that Russia was behind the Salisbury nerve agent attack.

In a statement that followed a significant US intervention at the United Nations in support of Britain’s position, the White House said it backed the Prime Minister’s decision to expel 23 Russian diplomats.

But French support was more equivocal with Emmanuel Macron’s spokesman declining to blame Russia in the same way until it had seen more evidence, though a later statement said France’s support was “unquestionable”.

The row over the 4 March attack involving a Russian-made “Novichok” nerve agent that left ex- spy Sergei Skripal and his daughter in a serious condition, is set to deepen today with Moscow planning how it will retaliate against Ms May’s actions, which also include asset freezes and new powers to tackle people planning “hostile activity”.

In a statement he White House stated: “The United States stands in solidarity with its closest ally, the United Kingdom.

“The United States shares the United Kingdom’s assessment that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support the United Kingdom’s decision to expel Russian diplomats as a just response.

“This latest action by Russia fits into a pattern of behaviour in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes.”

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

French government spokesman Benjamin Griveaux said on Wednesday that it was too early to decide on retaliatory measures as Russian involvement was yet to be proven.

But amid reports that France’s backing for Ms May’s approach was wavering, an Elysee source said: “France’s solidarity with the UK is unquestionable.

The United States shares the United Kingdom’s assessment that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter The White House

“President Macron denounced as early as Tuesday the Salisbury chemical attack as unacceptable and assured Prime Minister May the UK had France’s full support. Both leaders will discuss the matter yet again this Thursday.

“Since the beginning of this week, the UK has briefed its allies thoroughly, and France in particular, that it was highly likely that Russia was responsible for the attack.”

The US Ambassador to the UN, Nikki Haley, accused Russia of a “crime” worthy of action by the UN Security Council, as she backed Britain’s attempts to punish Moscow for the poisoning of a former Russian spy in Salisbury.

In an urgent meeting called by the UK Ms Haley told the Security Council the US believes “Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent”.

She said: “Alone, Russia’s crime is worthy of this council’s action, but this is not an isolated incident. The assassination attempt in Salisbury is part of an alarming increase in the use of chemical weapons”.

She added that American officials “take no pleasure in constantly criticising Russia,” but “it must account for its actions”.

On Wednesday Ms May moved to dismantle a Russian spy “network” in the UK, expelling 23 embassy officials in the biggest number of diplomatic expulsions since the Cold War.

The dramatic decision to remove “undeclared intelligence officers” came alongside measures to freeze Russian assets, cut off high-level communication with Moscow, pass new anti-espionage powers and isolate Vladimir Putin at the UN.