The European Union has offered Britain solidarity after London accused Russia of being behind a nerve agent attack on a former Russian double agent and his daughter in the English city of Salisbury.

The promise by Valdis Dombrovskis, the vice president of the European Commission, comes amid uncertainty in Britain regarding security cooperation after it leaves the EU at the end of March 2019.

When asked whether the EU might impose sanctions of Russia if it was agreed that Moscow was responsible for the attack, Dombrovskis said: "Of course, the UK can count on EU solidarity in this regard."

Read more: Spy assassinations — The top 5 deadly poisons 

A spokeswoman for EU foreign policy chief Federica Mogherini told Reuters news agency that the use of a military-grade chemical agent in a civilian's murder attempt on UK soil was "shocking."

"We stand with the UK in pursuit of justice in this case and are ready to offer support if necessary."

British Prime Minister Theresa May said on Monday that it was highly likely Moscow was to blame, as the substance had been identified as part of a group of nerve agents developed by the Soviet military in the 1970s and 1980s. She said her government would consider a "full range" of retaliatory measures if Moscow had not given a "credible response" by the end of Tuesday.

Read more - Novichok nerve agents – Russia's dangerous 'new' poison

UK allies respond:

• German Chancellor Angela Merkel said she took the British government's claim that Russia was involved in the poisoning "very seriously," and urged Moscow to provide "swift answers." 
• German Foreign Minister Sigmar Gabriel said: "It's clear the perpetrators must be held accountable. If it is confirmed that Russia is behind this, that would be a very serious matter."
• The French Foreign Ministry called the attack "totally unacceptable" and expressed its solidarity to "a top and strategic ally."
• US Secretary of State Rex Tillerson said Washington would stand by Britain and that Russia must face "serious consequences" if it was found to be behind the incident. Tillerson was ousted as secretary of state shortly after the announcement. 
• President Donald Trump later said he would speak with Theresa May on Tuesday about the situation. While Trump indicated that the evidence pointed to Russian involvement, the president said that as soon as facts are evaluated, "if we agree with them, we will condemn Russia or whoever it may be."

How has Russia reacted so far?

The Russian Foreign Ministry sharply criticized the UK's accusations in a statement, saying they were "openly provocative," and part of "yet another dirty attempt by British authorities to discredit Russia."

It added that Britain's threats of retaliatory actions "will not be left without a response."

The UK broadcasting regulator has warned that Russian state-owned channel RT could have its British license revoked if London concludes Moscow was behind the attack.

A Russian Foreign Ministry spokeswoman warned that Russia would not allow any British media outlets to operate on its soil if the license was canceled. 

"Not a single British media outlet will work in our country if they shut down Russia Today (RT)," spokeswoman Maria Zakharova was quoted by the state-run RIA news agency as saying.

She added that nobody should threaten a nuclear power.

Earlier, Foreign Minister Sergey Lavrov insisted Moscow was not to blame for the attack, and said Russia would cooperate with the investigation only if it received samples of the nerve agent believed to have made the ex-spy and his daughter ill. He said Moscow's requests to see samples had been turned down.

Russia's embassy in London called for a joint investigation into the incident.

What happened in the attack? Russian ex-spy Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench in the southern English city of Salisbury on March 4. They remain in critical but stable condition in hospital. A policeman who attended to them is also in critical condition, but conscious.

Who is Sergei Skripal? Skripal was a colonel in Russia's military intelligence service, the GRU, before he was found guilty of betraying Russian agents to MI6, Britain's foreign intelligence service. In 2010, he arrived in Britain as part of a spy swap deal between London and Moscow.

tj, jcg/rc (AP, Reuters, AFP)

Every evening at 1830 UTC, DW editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.