Poisoned ex-Russian spy Sergei Skripal reveals how he used invisible ink to leak secrets to Britain after being turned by MI6

FORMER Russian spy Sergei Skripal passed secrets to MI6 using invisible ink after being flipped by British intelligence after the collapse of the Soviet Union.

The informant’s decision to turn his back on the Kremlin led to the botched nerve agent attack in Salisbury in March, an explosive new book claims.

The Skripal Files, written by BBC journalist Mark Urban, details how he became disillusioned with his country’s GRU spy agency.

In 1996, Sergei was recruited by UK intelligence towards the end of his three-year stint working in Spain.

He had previously worked at the Russian embassy in Malta – where the sunny weather, luscious seafood and friendly locals had helped foster his disenchantment with Moscow.

Skripal told Urban that he had tried to resign from the GRU a year before arriving in Madrid after feeling betrayed by leaders Mikhail Gorbachev and Boris Yeltsin after the collapse of the Soviet Union in 1991.

The book tells of how the Russian family man was turned by a young MI6 agent who is referred to by his alias Richard Bagnall.

Under his agreement with British intelligence, he agreed to trade secrets for cash as long as he had an escape plan – in the form of documents and a passport.

He was MI6’s first agent inside the Kremlin in 30 years and was given the codename Forthwith.