Britain's accusations against Russia over the poisoning of a spy in Salisbury are "against international law and common sense" and the UK can expect Moscow's retaliation to come "at any moment", Vladimir Putin's spokesman has said.

The Kremlin is taking its time to develop a “considered” response to British sanctions, the presidential press-secretary Dmitry Peskov said on Friday. The Russian reaction would be made in line with its national interests, he said, and will be announced “very soon.”

Foreign Minister Sergei Lavrov has already signalled that 23 British diplomats will be asked to leave the country, mirroring the British move against Russia’s London embassy. But there has as yet been no other signal as to how Moscow will deal with the rest of the package announced by Theresa May on Wednesday. This included the promise to step up sanctions, a threat to freeze “hostile” Russian state assets, and a wider intention to work against “suspicious” Russian money in the UK.

Ms May said on Wednesday that the UK considered Russia responsible for the poisoning of the former spy Sergei Skripal, 66, and his daughter Yulia, 33, using a Soviet-era nerve agent in the cathedral town of Salisbury. In addition to ejecting 23 Russian diplomats who she accused of spying in Britain, she also cut bilateral ties and threatened further action against rich Russians in the UK.

Mr Peskov said that the Kremlin had been “surprised” by the British government’s actions. “We haven’t seen this type of behaviour on a state level before,” he told journalists. “Delivering serious accusations against […] Russia, on a ‘highly likely’ basis, is both against international law and common sense.”

The president’s spokesman played down the reality of an “apocalyptic” cold war developing between Russia and the West. He said Russia remained open for “friendly” relations with any country prepared to cooperate with Moscow.

But it will not be long before the Russians published their counter-measures against the United Kingdom, he said: “You can expect them at any moment.”

Britain, the US, Germany and France jointly called on Russia on Thursday to explain the attack. Donald Trump said it looked as though the Russians were behind it.

Russia has refused to explain how Novichok, a nerve agent developed by the Soviet military, was used to strike down the Skripals on 4 March. They were found unconscious on a bench and remain critically ill in hospital.

A British policeman, Detective Sergeant Nick Bailey, was also poisoned as he went to assist the pair. He is described as being in a serious but stable condition. Authorities have warned that dozens of other British citizens were put at risk by the attack, and more than 30 received medical checks and attention, though all were promptly discharged.

Thursday saw an exchange of openly insulting language between British and Russian ministers. UK Defence Secretary Gavin Williamson was criticised at home and abroad for saying that Russia "should go away, it should shut up".

Sergei Lavrov, the Russian Foreign Minister, said the comments suggested Mr Williamson "lacked education".