Russia has called the UK's allegations that the Kremlin may be responsible for the mysterious illness affecting a former Russian spy, "groundless".

Maria Zakharova, a spokesperson for Russia's foreign ministry, said on Tuesday the case of Sergei Skripal, a former Russian spy who collapsed on UK soil last Sunday, should be investigated before such allegations are made.

Skripal, a former colonel in Russia's foreign military intelligence agency, and his 33-year-old daughter, Yulia Skripal, were found unconscious on Sunday at a shopping centre in Salisbury in southern England before being taken to hospital.

British police said both individuals remain in "critical condition", though they bear no visible injuries.

Investigators are now examining CCTV footage showing the pair's last movements before they fell ill.

Boris Johnson, the UK minister of foreign affairs, said Skripal's condition "echoes" the death of ex-KGB agent Alexander Litvinenko in 2006.

Litvinenko was a former Russian spy who died in London after he was poisoned.

'Russian reaction'

Speaking in the UK House of Commons on Tuesday, Johnson warned that Britain would respond "appropriately and robustly" - such as by imposing new sanctions on Russia - if Russia is found responsible for Skripal, 66, falling critically ill.

Dmitry Peskov, Russian President Vladimir Putin's spokesperson, has called the incident "tragic".

"But we don't have information on what could have led to this, what he was engaged in," he said on Tuesday.

Russia will assist UK authorities with their investigation into the incident, Peskov said.

Mark Rowley, assistant commissioner of the UK's Metropolitan Police, told the BBC the investigation will involve Britain's "counterterrorism network" if deemed necessary.

"It's a very unusual case and the critical thing is to get to the bottom of what's caused these illnesses as quickly as possible," he said.

"We're doing all the things you would expect us to do: we're speaking to witnesses, we're taking forensic samples at the scene, we're doing toxicology work and that will help us to get to an answer. I can't say any more at this stage."

Skripal arrived in the UK in 2010 as part of a spy swap, which resulted in four prisoners being released by Moscow.

In return, 10 agents were released and sent to Russia from the US.

Skripal had been serving a 13-year jail term in Russia, following his initial arrest by the Federal Security Service (FSB) in 2004.

The FSB allege Skripal supplied information regarding Russian agents to British intelligence services.

The Litvinenko case

In November 2006, Litvinenko died three weeks after drinking green tea laced with radioactive polonium-210 at London's Millennium Hotel.

Russia denied involvement in his death.

A British inquiry into the 43-year-old's death said the Kremlin had "probably" ordered for Litvinenko, who lived exiled in Britain, to be assassinated.

The UK government, however, refused Judge Robert Owen's request for a public hearing into Litvinenko's death, following the inquiry.

Russia's foreign ministry dismissed the inquiry as "biased" and "opaque", according to the official RIA news agency.