LONDON - Britain and its allies on Thursday pointed the finger directly at Moscow over the poisoning of a Russian double agent, as the Kremlin vowed prompt retaliation against the “irresponsible” expulsion of its diplomats from London.

The escalating international scandal is unfolding as former Russian double agent Sergei Skripal and his daughter Yulia remain in critical condition after exposure to the Soviet-designed chemical Novichok on March 4 in the English city of Salisbury.

British Prime Minister Theresa May promised a “united stance” as she visited the scene of the attack for the first time on Thursday, a day after saying Moscow was “culpable” and expelling 23 Russian diplomats among other measures.

In a rare joint statement, the leaders of Britain, France, Germany, and the US condemned the attack as an “assault on UK sovereignty”.

“We share the UK assessment that there is no plausible alternative explanation, and note that Russia’s failure to address the legitimate request by the UK government further underlines its responsibility,” the statement said.

The four leaders called on Russia to provide “full and complete disclosure” of the Soviet-era chemical programme that developed Novichok, the statement published by the British government said.

Russia did not meet London’s demand that it disclose details of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons (OPCW) by midnight on Tuesday, leading May to announce the retaliatory measures. Kremlin spokesman Dmitry Peskov called Britain’s position “absolutely irresponsible”.

He warned that retaliatory steps would soon follow and President Vladimir Putin would choose the option that “most suits Moscow’s interests”.

Russia would respond by kicking out British diplomats, Foreign Minister Sergei Lavrov was quoted as saying in Moscow, adding that it would happen “soon”.

He said Moscow did not have a motive to attack Skripal but suggested other players could use the poisoning to “complicate holding the World Cup” in Russia this year and charged that the British government is keen to “deflect attention” from its troubles with Brexit.

May’s statements that Moscow is behind the poisoning are “completely crazy accusations against Russia, our entire country, our people,” foreign ministry spokeswoman Maria Zakharova told journalists.

Moscow has mostly shrugged off the measures announced by May, ridiculing the decision not to send British ministers and royals to the World Cup.

Zakharova further accused Britain of refusing to grant access to Yulia Skripal or work with Moscow through the OPCW and give Russia access to the poisonous substance.

However British Foreign Secretary Boris Johnson told the BBC that Britain is “entirely in conformity” with OPCW procedures and is sending a sample of the nerve agent to the watchdog for examination.

The US also told an emergency meeting of the UN Security Council that it believed Russia was responsible for the use of the military-grade nerve agent against Skripal.

NATO allies have branded the attack a “clear breach of international norms and agreements” and Secretary General Jens Stoltenberg said it comes against the “backdrop of a pattern of reckless behaviour” by Russia.

He said however that Britain had not invoked the alliance’s Article 5 mutual defence clause.

French President Emmanuel Macron said he would announce unspecified “measures” to respond to the poisoning in the coming days.

Johnson further appealed to international partners to “stand with us against Russia” in a Washington Post editorial, saying the poisoning is “part of a pattern of reckless behaviour” by Putin showing “reckless defiance” of international rules. He accused Moscow of using assassinations to send a signal to dissidents that “we will find you, we will catch you, we will kill you.”

Putin, who is standing in a presidential election Sunday, has not yet commented on Britain’s measures but discussed the Skripal situation with his security council Thursday. His spokesman said the poisoning row had no effect on the campaign.

Besides expelling the diplomats - the biggest such move in 30 years, suspending high-level contacts and cracking down on Russian criminals and dirty money, Britain will invest in a new chemical weapons defence centre, said Defence Secretary Gavin Williamson.

The centre will receive £48 million (54 million euros, $67 million) of investment and will be based at the existing Porton Down secretive base.

Russia argues that it has destroyed all of its chemical weapons and that its military chemical programme has been shut down since the mid-1980s.

Russian chemist Vil Mirzayanov, who worked in the chemical weapons programme until 1992 and had exposed the Novichok agents prior to leaving for the United States, however claimed Moscow had stored the substance and its authorities “are still keeping it in secrecy”.

Mirzayanov said Russia was likely behind the attack, though adding that it was also possible that somebody used the Novichok formula published in his book to synthesise the chemical.