The “strength of the support” shown to the UK over the poisoning of former Russian agent Sergei Skripal and his daughter has been encouraging, the foreign secretary has said.

Boris Johnson said the UK had been talking to friends and there had been a “willingness” to show “solidarity”. Russia’s foreign minister said claims of Russian involvement were “rubbish”.

The UK has given Russia a midnight deadline to explain why a Russian-made nerve agent was used in the attack. Russian Foreign Minister Sergei Lavrov said that Russia had been refused access to the substance that was used to poison Mr Skripal and it would not respond to the ultimatum until it was given access.

The UK ambassador to Russia, Laurie Bristow, has been summoned to the Russian Foreign Ministry, according to Russian news agency Interfax.

Former double agent Mr Skripal, 66, and his daughter, Yulia, 33, were found slumped on a bench in Salisbury city centre in Wiltshire on 4 March. They remain in a critical but stable condition in hospital.

Det Sgt Nick Bailey, who fell ill attending to the pair, remains seriously ill, but has been talking to his family.

Home Secretary Amber Rudd has chaired another meeting of the government’s emergencies committee Cobra to discuss the case.

Prime Minister Theresa May told the Commons on Monday that the poison used in the attack was a military-grade nerve agent developed by Russia. She said it was part of a group of nerve agents known as Novichok. “Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others,” she said.

Mrs May said the Foreign Office had summoned Russia’s ambassador to “explain which of these two possibilities it is”. She warned that if there was no “credible response” by the end of Tuesday, the UK would conclude there has been an “unlawful use of force” by Moscow.

Nato secretary-general Jens Stoltenberg said the UK was a “highly valued ally” and described the incident as “of great concern”. He said the use of any nerve agent was “horrendous and completely unacceptable” and said Nato had been in touch with the UK authorities.

US Secretary of State Rex Tillerson said it appeared the “really egregious act… clearly came from Russia” and there should be “serious consequences”. Mr Tillerson, who spoke to Mr Johnson on the phone about the case on Monday, said the US supported the UK’s assessment that Russia was likely responsible. He added: “We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences. “We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses.”

Mrs May also spoke to French President Emmanuel Macron on Monday and the two leaders “agreed that it would be important to continue to act in concert with allies” to address what it called “the wide pattern of aggressive Russian behaviour”, her spokesman said.

European commissioner Valdis Dombrovskis was among a number of leading EU figures to express “solidarity” with the UK. He said: “We are very much concerned with this situation – also with the findings the UK has so far.”

Guy Verhofstadt, European Parliament’s Brexit co-ordinator, wrote on Twitter: “We stand shoulder to shoulder with the British people. “It must be made clear that an attack against one EU and Nato country is an attack on all of us.”

How Could The UK Retaliate Against Russia?

Mrs May said the UK must “stand ready to take much more extensive measures” against Russia than it had previously. She said these measures would be set out in the Commons on Wednesday should there be no adequate explanation from Russia.

Britain could expel Russian diplomats, as it did after the poisoning of former Russian Federal Security Service operative Alexander Litvinenko in 2006 with radioactive polonium. But many argue that this, and the other measures that were taken after that killing – including visa restrictions on Russian officials – did not go far enough.

Other possible actions could include:

Freezing financial assets
Bans on visas
Boycotting the Fifa World Cup in Russia later this year
Taking Russian broadcasters such as RT (formerly Russia Today) off the air in the UK