The nerve agent that poisoned the Russian spy Sergei Skripal was planted in his daughter’s suitcase before she left Moscow, intelligence agencies now believe.

Senior sources have told the Telegraph they are convinced the Novichok nerve agent was hidden in the luggage of Yulia Skripal, the double agent’s 33-year-old daughter.

They are working on the theory that the toxin was impregnated in an item of clothing or cosmetics or else in a gift that was opened in his house in Salisbury, meaning Miss Skripal was deliberately targeted to get at her father.

At a public meeting on Thursday evening Paul Mills, deputy chief Constable of Wiltshire police, revealed 131 people could have potentially come into contact with the deadly nerve agent, and that they were being monitored by health authorities over the phone on a daily basis.

He also said 46 people had attended hospital expressing concerns since the incident, and that cordons around areas where traces of the nerve agent had been found or could yet be found may remain in place for months.

Col Skripal was convicted of spying for Britain in 2006 but came to the UK in 2010 in a spy swap.

Counter terror police and MI5, hunting the would-be assassins, no longer think the Kremlin-backed hit squad ever entered the UK, making it much harder for the UK authorities to pinpoint exactly who carried out the attempted murder of Colonel Skripal, 66, and his daughter. They remain in intensive care, fighting for their lives.