THE nerve agent used in the botched assassination of Sergei Skripal and his daughter Yulia was reportedly designed to take four hours to kill them — so their attackers could escape the country.

A "boutique" version of deadly Russian-made Novichok was developed in a gel form to be smeared on the door handle of the ex-Soviet double agent's home in Salisbury, Wilts, it is claimed.

Security sources have told the Mail On Sunday the chemical weapon is usually created as a gas which can kill in minutes.

But President Putin's scientists were tasked with designing a less toxic version of Novichok that can be spread over household objects and absorbed through the skin.

One source told the paper: "The Kremlin wanted to get its agents out of Britain before the Novichok could be identified.

"The Russians still banked on Sergei and Yulia dying as a result of their exposure, even though they had effectively watered it down."

The source added: "Their recovery has been received with great disappointment in Moscow."

While buying their would-be hitmen time to flee, the lowered toxicity allowed doctors time to diagnose and treat Sergei, 66 and Yulia, 33.

Hamish de Bretton-Gordon, the former commander of Britain’s Chemical, Biological, Radiological and Nuclear Regiment, last night praised brave medics for their ability to "thwart a boutique chemical weapon".

It emerged that two doctors at Salisbury District Hospital had only recently completed specialist chemical weapons training at Britain's chemical warfare lab Porton Down.

They were able to quickly determine the symptoms of a nerve agent attack and administer the atropine antidote, the MoS said.

Russian agents had planned their attack for two weeks and decided to strike on a Sunday so no postmen or delivery men would be exposed accidentally to the nerve agent, the report added.

Recovering Sergei and Yulia - who is now thought to be close to being discharged after saying she is gaining strength daily - believe Russia will try to "finish the job" and kill them.

They will be given new identities and a new home in Britain following the March 4 attack. Russia denies any involvement.