LONDON: British Prime Minister Theresa May was expected to announce retaliatory action against Russia on Wednesday after Moscow rejected her "unfounded accusations" over the poisoning of a Russian former double agent on UK soil.
Relations between the two countries have deteriorated rapidly in the 10 days since ex-spy Sergei Skripal and his daughter Yulia were attacked with a nerve agent in sleepy Salisbury, southwest England.
The British premier said on Monday it was "highly likely" that Russia was behind the attack, either directly or because Moscow had "lost control" of the nerve agent, demanding answers by the end of Tuesday and threatening a "full range" of measures.
But Russia defied the deadline and President Vladimir Putin's spokesman on Wednesday rejected Britain's "unfounded accusations that are not based on evidence, and a language of ultimatums".
"We are hoping that common sense will prevail," Dmitry Peskov told reporters.
May is due to address parliament in the early afternoon after holding a meeting with her National Security Council to discuss the government's next move.
'Hitting economic targets'
The former director of Britain's communications spying agency GCHQ, Robert Hannigan, said her response should include "the expulsion of diplomats on a scale we probably haven't seen since the Cold War".
Speaking to BBC radio, Hannigan also backed "hitting the economic targets", including Russians doing business in London, but warned against a large-scale cyber attack.
Britain is wary of acting alone and has been rallying support from its allies.
"She believes that Russia is seeking to undermine the international rules-based system and that is something that we and are allies are obviously determined to defend," May's spokesman said on Tuesday.
In a phone call with US President Donald Trump, he and May "agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms", the White House said.
Foreign Secretary Boris Johnson on Tuesday spoke to his counterparts in France and Germany, as well as NATO Secretary General Jens Stoltenberg.
If Russia is found responsible, "this would be further reckless behaviour which threatens the international community and requires an international response," Johnson told them, according to the Foreign Office.
The spy saga comes at a particularly tense time for UK-EU relations, as the two sides are locked in Brexit talks, but Brussels' support is "unequivocal, unwavering and very strong" according to European Commission Vice President Frans Timmermans.
Stoltenberg said the incident was "of great concern" amid reports that Britain was consulting NATO allies about possibly invoking the alliance's Article 5 principle of common defence.
Russian chemist blames Moscow
As Britain prepares its response, Skripal, 66, and his daughter Yulia, 33, remain in critical condition in hospital.
British experts say they were poisoned with military-grade nerve agent from a broad category known as Novichok, developed by the Soviet Union during the late stages of the Cold War.
The Russian chemist who first revealed the existence of Novichok, Vil Mirzayanov, said "only the Russians" developed the Novichok agents.
"They kept it and are still keeping it in secrecy," he said from his home in the US, where he moved in 1995 after 30 years of working for the State Scientific Research Institute of Organic Chemistry and Technology.
Moscow has denied any involvement, with Foreign Minister Sergei Lavrov insisting the Kremlin is ready to cooperate with Britain but complaining that its request for samples of the nerve agent had been rejected.
Dire relations between London and Moscow are not without precedent, with diplomats being expelled numerous times including in 2006 over the poisoning of former agent Alexander Litvinenko.
Anticipating such a move, the Russian embassy in London warned on Twitter of retaliatory measures.
"Any threat to take 'punitive' measures against Russia will meet with a response. The British side should be aware of that," it said.
May on Monday hinted at legislating to make it easier to seize the British assets of Russian officials suspected of human rights abuses, which could hit some of the super-rich Russians who have invested heavily in the London property market in recent years.
Imposing a boycott by British officials and royals of this summer's World Cup in Russia, while allowing the England team to play, is another possible response.
The first sign of action came from British communications regulator Ofcom, which said it could review the licence of the Kremlin-backed RT broadcaster if Russian involvement in the poisoning is proven.
Russia, in turn, threatened to ban all British media if RT loses its licence.
In a further twist, former senior Russian executive Nikolai Glushkov, linked to late Kremlin opponent Boris Berezovsky, was found dead in London on Tuesday in unexplained circumstances, British and Russian media reported.