Theresa May is to hold a three-way crisis meeting on the Salisbury nerve agent attack with the French and German premiers in the margins of a European summit on Thursday.

The Prime Minister is attending the European Council meeting in Brussels where she will brief the other 27 EU leaders on Brexit and the Salisbury nerve agent incident, as well as consider a response to rising US trade protectionism.

Arriving at the Europa building in the Belgian capital Ms May told reporters that Russia had “staged a brazen and reckless attack against the United Kingdom” by using the Novichok nerve agent.

“I'll be raising this issue with my counterparts today because it's clear that the Russian threat does not respect borders, and indeed the incident in Salisbury was part of a pattern of Russian aggression against Europe and its near neighbours, from the western Balkans to the Middle East,” she said.

Downing Street said the Prime Minister was set to meet the three premiers in the early evening before the 28 EU leaders sit down for dinner.

Federica Mogherini, the EU’s foreign affairs chief, said EU leaders would “express our strongest possible solidarity with the United Kingdom after the attack in Salisbury, as well as we have done with the foreign ministers on Monday”.

“The strongest political sign we can give is unity, unity, and unity: including in the moment when we move forward on negotiations on Brexit,” she said on the doorstep of the meeting.

EU foreign affairs chief Federica Mogherini speaks at the summit (EAS)

“This doesn’t mean anything in terms of diminishing our solidarity, on the contrary we stand together.”

EU diplomats ahead of the summit said Britain was not pushing for new sanctions against Russia, but that the focus had instead been getting EU leaders to accept the UK position Russia was behind the attempted poisoning.

Investigations by military experts at the Defence, Science and Technology Laboratory at Porton Down identified the nerve agent used against the Skripals as part of the Soviet-developed “Novichoks” group.

Independent experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) are continuing work to verify the analysis in Salisbury and have been seen in areas visited by Mr Skripal and his daughter.

The results are expected to take a minimum of two weeks and police have not confirmed how the nerve agent was administered.

NHS England said a total of 38 people were treated for varying symptoms resulting from exposure to the substance, but only the Skripals and Detective Sergeant Nick Bailey remain in hospital, with an unnamed person being seen as an outpatient.

A judge gave doctors permission to pass fresh blood samples from the victims to the OPCW on Thursday.

Mr Justice Williams made the ruling in the Court of Protection because the Skripals remain unconscious in intensive care, and are unable to give consent.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

The judge ruled that the Salisbury NHS Foundation Trust could provide blood samples to the OPCW alongside copies of medical notes, saying the moves were in their best interest.

The Russian government has promised to cooperate with the OPCW but dismissed its conclusions that ally Bashar al-Assad killed civilians with sarin gas in Syria last year.

It denies any wrongdoing and claims it has destroyed all chemical weapons stockpiles and complied with the Chemical Weapons Convention.

Alexander Yakovenko, the Russian Ambassador to the UK, heightened tensions by accusing Britain of storing deadly Novichok itself in an extraordinary press conference.

The Prime Minister said Russia had provided “no credible explanation” on how the substance came to be used on the streets of Wiltshire and accused the state itself of culpability.

A meeting of EU foreign ministers on Monday did not explicitly say Russia was responsible for the attack, and said only that the “European Union takes extremely seriously the UK Government’s assessment that it is highly likely that the Russian Federation is responsible”.

It is understood that UK diplomats have been pressing for national leaders to go further in their condemnation than foreign ministers did and explicitly point the blame at Russia.