Experts fear poison may have been concealed in a gift

added: “The public should not be alarmed and the public health advice remains the same.”

On Wednesday, Dame Sally Davies, the UK’S chief medical officer, said there was no apparent wider health risk.

Both Col Skripal, an officer in the Russian military intelligence who was convicted of spying for MI6, and his daughter remained critically ill last night. He came to Britain in 2010 in a spy exchange.

Asked if there were any leads in the case, Lord Blair, the former Metropolitan Police Commissioner, said in a BBC interview: “Clearly what they’re trying to find out at the moment is how was this [nerve agent] delivered personally. There obviously are some indications. The officer... has actually been to the house, whereas there’s a doctor who looked after the patients in the open who hasn’t been affected at all.”

The Daily Telegraph has independently confirmed that Det Sgt Bailey did attend the house. That points to the nerve agent being administered there rather than in the street where the Skripals collapsed, as previously thought.

Counter-terrorism police are now investigating whether Yulia Skripal, 33, inadvertently brought a gift for her father from Moscow that contained the nerve agent, planted by Russian intelligence in her luggage.

Experts told The Telegraph they were baffled at the possibility that the Skripals may have been exposed to a nerve agent at their home, only to be taken ill some time later.

Hamish de Bretton-gordon, a former commander of the British Army’s chemical, biological, radiological and nuclear regiment, said it would be “very strange”, adding: “Nerve agents usually take seconds or minutes.”