Single agent: John Bingham, the real-life inspiration for John le Carré's George Smiley

John Bingham was John le Carré's mentor - and the inspiration for le Carré's most famous creation, but his biography is a rather bloodless affair.

Gary Oldman as George Smiley in the spy thriller Tinker, Tailor, Soldier, Spy.

"Small, podgy, and at best middle-aged, he was by appearance one of London's meek who do not inherit the earth. His legs were short, his gait anything but agile, his dress costly, ill-fitting, and extremely wet."

Not someone who would leave an impression if you passed him on the street. But then that's the point: to fade into the shadows. The character, of course, is George Smiley, the inimitable creation of spy novelist John le Carré. Le Carré's Smiley novels, saturated in ambiguity and treacherous dealings, perfectly captured the unease of the Cold War years in the West. Smiley may not have been the perfect spy, but he was the man for the hour.

Over the years, le Carré (born David Cornwell) has unveiled the real-life models for his fictional creation. One was the Reverend Vivian Green, whom the young Cornwell met at public school in Dorset. (Green was also later Cornwell's tutor at Lincoln College, Oxford.) The other inspiration, John Bingham, had a direct connection to the world of secret intelligence. Bingham was an operative at MI5, and worked with le Carré briefly in the late 1950s. He mentored him, but their relationship was put to the test once le Carré's literary career took flight.

Bingham's life is an inviting subject for a biographer. The son of an Anglo-Irish baron - his father was the 6th Baron Clanmorris - Bingham (1908-1988) joined the Security Service in 1940 after a spell on Fleet Street. An almost comically strait-laced figure - Cornwell dubbed him "le Carré", the square, the coinage then becoming part of the famous pen name - Bingham proved masterful at his craft, handling his agents with skill. He was a keen listener and stringent interrogator, possessed of both empathy and objectivity. He could coolly appraise a situation and manoeuvre himself out of a tight spot. Bingham also had a second career as a writer of crime thrillers, several of which have been republished. And he knew how to compartmentalise: so secretive was Bingham about his work that for many years his own literary agent did not know he worked for MI5.

It's a surprise, then, that Michael Jago's biography of Bingham falls so flat. Jago is a diligent researcher, but a tepid writer. This is an account that's as strait-laced, traditional, and conventional as its subject. Separating the fact from fiction when writing about the clandestine services is a difficult task; but there is precious little texture or drama in what otherwise might have been an exciting life story.

Bingham was highly capable and beloved by his agents. In his work for MI5, he played cat and mouse games with the Abwehr during the Second World War, as well as travelling throughout Britain to investigate allegations of activity by enemy agents. He kept tabs on Britain's communists during the Cold War. He tracked subversive elements in the arts world. In the early 1970s, he looked into right-wing plots against the Labour prime minister Harold Wilson. A conservative, Bingham saw in extremes of left and right threats to the established order.

The chapters on Bingham's relationship with le Carré - whose 23rd novel, A Delicate Truth, will be published next month - are the book's most successful, and will surely be of interest to fans for the light they shed on evolution of Smiley

Bingham, observes Jago, "was a resolute, self-confident defender of Britain's character". He was also "an avuncular figure, a man who took great pleasure in encouraging men younger than himself". These two dispositions were put to the test over the decades he knew le Carré.

It would prove a fraught, though friendly, relationship. Bingham introduced le Carré, then a budding writer, to his agent and publisher. Le Carré, however, vastly eclipsed Bingham in the literary field, becoming a writer of the very first rank, whose spy thrillers transcend the narrow confines of the genre. Le Carré's views troubled his mentor. Bingham was an ardent defender of the Security Service, whereas Le Carré took a much darker view: he suggests a rough moral equivalence between the West and the other side. There is really no just cause. Bingham believed in loyalty to the Security Service; le Carré thought otherwise, and criticised the intelligence professions in print and elsewhere.

It also got personal. Bingham's wife Madeleine took fierce exception to a comment le Carré made in the late 1970s, when he described Smiley as "forgettable", and launched a campaign of vituperation against her husband's perceived antagonist. Yet, as Jago points outs, being forgettable is a most valuable, if not essential, trait for an operative. Le Carré insisted he meant it as a compliment - as he saw it, his Smiley was an affectionate reflection of Bingham.

For his part, Bingham reached out to le Carré in a letter flecked with humility, whimsy, and poignant loyalty to the Security Service. "Smiley has never minded being called a Forgettable Man," Bingham wrote, "and has even regarded it as a mild professional compliment to a man who can merge into the general background. He thinks that that and keeping a low profile are quite useful traits.

"On the other hand, to be perfectly frank - always difficult for Smiley - he has often been puzzled as to why you have so frequently and harshly attacked his mob, directly in interviews or obliquely in books. (Troops don't normally improve by, in effect, being called a lousy lot of bums, and inefficient or ineffective to boot.)

"You are far from being pro-Soviet Russia or pro-Communist, but I would think the attacks gave comfort and even pleasure and glee in some places. Puzzling to simple souls like Smiley - perhaps his only really unsolved mystery."

Jago's take on such proceedings is measured and fair. Le Carré was from a different generation, and his views contrasted sharply with those of Bingham. But the author imagines that le Carré must have had something like a eureka moment when he joined MI5 in 1958 and met Bingham. Here was the very model for Smiley: "The son of a baron, bemused by the changing society around him, dedicated to his job, supremely confident of the rightness of his cause, yet analytical enough to have reached his ethical posture logically."

He notes that Bingham's children were amazed by the resemblance between their father and Alec Guinness's Smiley in the BBC production of Tinker, Tailor, Soldier, Spy. "From the simple foibles, such as polishing his spectacles on the fat end of his tie, to his gait, facial mannerisms, deep, mellifluous voice and his ability to withdraw, to vanish into a crowd, this George Smiley was remarkably close to their perception of the original."

Of all the protagonists in this little drama, it was Madeleine who proved the most ferocious. Jago writes: "Bingham was able to depersonalise the rift, unlike Madeleine, whose fulminations against le Carré became increasingly personal." Bingham detested "everything he has done and said against the intelligence services", but, as Jago nicely observes, he was able to view le Carré with the kind of humane detachment that was the hallmark of George Smiley.

Literary creations emerge from a complex welter of emotion, instinct, imagination and subconscious reflection. In Bingham, le Carré found the perfect model for his fiction, whatever the real-life fallout.

Matthew Price's writing has been published in Bookforum, the Los Angeles Times, The Boston Globe and the Financial Times.