The Russian embassy in the UK reportedly plans to ask for consular access to the poisoned daughter of Sergei Skripal, the former spy attacked using a nerve agent on British soil.
It comes as Moscow’s foreign minister, Sergei Lavrov, accused Britain of attempting to mislead the international community over the case.
Mr Skripal and his daughter, Yulia, remain in critical condition in hospital after being exposed to the military-grade nerve agent Novichok last week.
Earlier on Wednesday morning, a Kremlin spokesperson said Russia was in no way connected to the incident in Salisbury, and that Moscow remained open to cooperating with a British investigation into the case.
Dmitry Peskov told reporters Russia “rejects the language of ultimatums” after Theresa May gave it until the end of Tuesday to explain how the Soviet-made nerve agent came to be used to target the former spy.
Mr Peskov said Britain has so far only offered “baseless accusations which are not backed up by any evidence”.
But the US, European Union and Nato voiced support for the UK after Ms May said it was “highly likely” Russia was behind the poisoning.
World news in pictures
The Government’s response to the expiry of the deadline is expected to be announced by Ms May in Parliament later.
Options include freezing the assets of Russian business leaders and officials, limiting their access to London’s financial centre, expelling diplomats, and even launching targeted cyber attacks.
England’s participation in the World Cup – which Russia is hosting in June and July – may also be cut back.
Russia is due to hold a presidential election on Sunday in which Vladimir Putin, himself a former KGB spy, is expected to coast to a fourth term.
Donald Trump told Ms May Moscow “must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom,” the White House said.
The White House said Mr Trump and Ms May “agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms”.
A British readout of the conversation said: “President Trump said the US was with the UK all the way.”
Additional reporting by PA