Russia will refuse to meet British Prime Minister Theresa May's midnight deadline unless Britain agrees to send Moscow samples of the nerve agent used to poison Sergei Skripal.

Russia’s embassy in the UK fired off a salvo of seven tweets in which it said Britain must comply with the Chemical Weapons Convention for a joint investigation and warned the threat of sanctions would "meet with a response".

It comes as US President Donald Trump told Mrs May in a phone call the US is "with the UK all the way", according to Downing Street.

Mrs May said the government had concluded it is "highly likely" Russia was responsible for the attack which left ex-spy Mr Skripal and his daughter Yulia in a critical condition in hospital.

Investigations continue at the scene of Salisbury 

She demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out measures Britain will adopt if no credible response is received by the end of today.

Germany and France also expressed solidarity with Britain today but beyond strong vocal EU support, diplomats said there was little appetite for more economic sanctions on Russia.

German Chancellor Angela Merkel spoke to Mrs May today, a day after French President Emmanuel Macron, and both leaders condemned the attack in statements today.

"[Merkel] said it's up to Russia to quickly provide answers to the British government's justified questions and to heed the call to completely and immediately lay bare the relevant chemical weapons programme to the Organisation for the Prohibition of Chemical Weapons," said Ms Merkel's spokesman Steffen Seibert, referring to the watchdog in The Hague.

Tánaiste and Minster for Foreign Affairs Simon Coveney condemned the attack, and said that Ireland has joined in supporting the UK's efforts to ensure a thorough investigation so that the perpetrators of the crime can be held accountable. 

British Home Secretary Amber Rudd earlier said Russia had "started responding" but the embassy appeared to suggest this amounted to little more than informing the Foreign Office of its demands and reiterating it was not involved.

Britain's ambassador to Russia Laurie Bristow was summoned by Moscow and told by first deputy foreign minister Vladimir Titov that the Kremlin "strongly protested" the accusations, the embassy said.

UK police said the probe could take several weeks as investigators carry out a "painstaking" operation to identify how the Novichok nerve agent was used to poison Mr Skripal.

Counter-terror police chief Neil Basu said officers' "prime focus" is to establish the method used to administer the chemical weapon.

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A huge police inquiry was launched after the former double agent, 66, and his daughter, 33, were found slumped on a bench in Salisbury, Wiltshire, on 4 March.

Detective Sergeant Nick Bailey, who was part of the initial police response, was also taken ill. He is in a serious but stable condition.

Speaking at Scotland Yard, Mr Basu warned the public in the city they could expect to see more police activity, saying: "In truth it may last many weeks."

The Metropolitan Police Assistant Commissioner would not identify any potential suspect, saying: "It's a painstaking operation to identify anyone of interest to this inquiry, and eliminate them or include them, but at this stage we are not declaring a person of interest or a suspect."

Police have so far collected 380 exhibits and have been scouring hours of CCTV footage from across the city.

Detailing the timeline leading up to the pair being taken ill, Mr Basu disclosed that Yulia had arrived at Heathrow Airport on a flight from Russia the day before.

The senior officer referred to Mr Skripal as a British national and Yulia as a Russian citizen.

He also revealed that a total of 38 people had been seen by medics in relation to the incident, of whom 34 had been assessed and discharged from hospital.

Three people remain in hospital - the Skripals and Mr Bailey. One other unnamed person is being monitored as an outpatient, but is not showing signs of exposure to the chemical weapon.