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A medic dubbed Dr Novichok accompanied poisoners who struck in Salisbury, investigators believe.

Security services are pursuing a doctor and a fourth man thought to have protected two Russian hitmen who targeted ex-spy Sergei Skripal .

The revelation comes days after the identities of the men suspected of the failed hit were given as Ruslan Borishov and Alexander Petrov – though they are believed to be assumed names.

The Sunday Mirror can reveal authorities are keeping a close hold on details of the other two suspects.

(Image: PA)

(Image: PA)

A Whitehall insider said the medic may have been involved to prevent cross-contamination – or even to make sure the NHS and police did not need to be involved if one of the hitmen was accidentally poisoned.

The source said Petrov and Borishov – who claim to be tourists – could have met up with the other two suspects after checking into the City Stay Hotel on Bow Road in East London.

They later caught trains from Waterloo to Salisbury, where the deadly Novichok was daubed on the door of the house where Skripal and daughter Yulia lived.

The source said: “It’s believed they could have met a handler who was working at the embassy and a fourth, the medic, might have had oversight of the whole thing to make sure they didn’t risk ruining the operation by getting themselves contaminated.”

(Image: RT) (Image: RT)

Other sources yesterday said police know the real ID of Petrov and Borishov.

An insider said: “They’ve had hundreds of officers trawling CCTV and the facial recognition software meant they could match them with passports used in the years leading up to the attack.”

Ex-spy Sergei, 67, and Yulia, 34, were poisoned but made a full recovery.

But Dawn Sturgess, 44, died later after handling poison dumped in a perfume bottle.

The Metropolitan Police said: “The investigation remains ongoing and it would therefore be inappropriate to discuss specific lines of enquiry.”