London Match — Len Deighton

Bernard Samson suspects there is a traitor within his
department of MI6. A jaded but highly skilled British intelligence agent
nearing the end of his career, Samson already got KGB major Eric Stinnes to defect.
But when a British KGB agent makes a sweeping confession with a suspicious
undertone, the finger points straight back to London -- where Stinnes is locked
up, refusing to talk.

The spy who's in the clear doesn't exist. In the spec­tacular
third novel in Len Deighton's Game, Set, Match trilogy, will Samson make
the winning move?