A THIRD Russian spy suspected of being involved in the nerve agent poisoning of the Skirpals in Salisbury might still be INSIDE the UK, it was feared last night.

A military intelligence officer using the likely fake name of Sergey Fedotov was booked on the same flights to and from Britain used by the two known would-be assassins.

But he suddenly checked himself and his baggage off the Moscow-bound plane shortly before departure, and it’s not known when – or if – he left the country.

One possibility is that Fedotov, whose passport said he was 45, remained in the UK after discovering Sergei Skripal, 67, and daughter Yulia, 33, had survived the attack.

A source told The Telegraph: “It is not clear why Fedotov did not board the flight.

"But at the last minute he checked himself and his bags off it.

"He could still have been running around Britain.”

Fedotov’s fake passport number was only a few digits different from the ones used by Russian intelligence agents Alexander Mishkin and Anatoliy Chepiga – who were also travelling under false names.

The senior GRU agents had been awarded Russia's highest honour for services in Ukraine.

They were captured on CCTV stalking the streets of Salisbury on the day of the attempted hit - but there are no known images of Fedotov.

His true identity has not yet been discovered, and it’s not clear what role he played in the attempted hit of Sergei Skripal – a former GRU intelligence officer who betrayed the Kremlin by selling secrets to MI6.

Some experts believe Fedotov could have brought the Novichok nerve agent into the UK.

Or he may have carried out secret reconnaissance to let the others know of Skripals’ movements.

The revelation Fedotov was supposed to be on the same March 4 flight as the other two Russian agents was broken by respected Russian news website Fontanka.

Fedotov previously travelled to the UK in March 2016 and March 2017 and again in 2018, according to the site.

Although the Skripals miraculously survived the nerve agent poisoning, they’re likely to have been left with health problems as a result.

And Brit Dawn Sturgess died after inadvertently spraying herself with the substance, which had been disguised in a perfume bottle and discarded by the hitmen.