I hate the idea of a "new Poirot", however good Sophie Hannah's announced post-mortem sequel turns out to be.

The reason I hate the idea of this boiled-over Agatha Christie is that the story of the "last Poirot" is so moving, and such a credit to the queen of crime as a person. The new book (and let's face it, money is the driving motive) will muddy one's sense of the dignified way she wrapped up the life of Hercule Poirot.

The long birth of Christie's last Poirot novel, the detective's curtain call, is well known. In the early days of world war two, Christie (who had done her bit as a nurse in the first world war), like many other brave Londoners, refused to leave the capital, while at the same time fully realising that she might die in the Blitz.

It was a point of honour. In those uneasy months it must have seemed if not the end of the world, then the end of old England – her world. The England, that is, of Styles and St Mary Mead, as celebrated in the genteel "tea-cosy" crime fiction of which, by 1939, she was the acknowledged monarch. There were others – Elizabeth Bowen, Stephen Spender, Virginia Woolf (a band of patriot authors chronicled in Lara Feigel's Love Charm of Bombs), who declined to follow Aldous Huxley, Christopher Isherwood and WH Auden to the safety of America. Christie was certainly internationally famous enough to have cut and run.

Expecting the worst, as many did in 1939, Christie wrote her message-in-a-bottle for posterity in the shape of a valedictory Poirot and Miss Marple novel. Having written them, by hand, the manuscripts were placed in a suitably bomb-proof safe, "in anticipation of my being killed in the raids, which seemed to be in the highest degree likely, as I was working in London," as she serenely put it. Hitler would have no easy victory over Christie (the Nazis favourite crime writer was Dennis Wheatley, author of The Devil Rides Out. Hermann Goering had some wild idea he might serve as a Gauleiter once England was occupied

).

The "last" Poirot novel was Curtain . It would be the detective's last case. Invalid throughout, he dies in the last chapter.

Happily, Christie's apprehensions about the war proved unfounded. Miss Marple and Poirot (now about 130 years old by pedantic count) were permitted to live on and do their bit to keep the world safe for England's decent, then unsqueezed, middle classes. Christie turned out a dozen or so postwar Poirots. They were snapped up by ever more readers – none of whom knew that the Belgian detective with the funny Edwardian moustache was dead – his remains lying peacefully interred in a bomb-proof safe.

In the mid-1970s, Dame Agatha, now in her 80s and ennobled for her services to literature, decided it really was time to draw the curtain, to carry out Poirot's 35-year belated obsequies. Curtain was finally published. Exit Poirot.

The full stop that Christie placed after her hero's long story in 1975 should be respected. It's not an iron rule. Some "franchise fiction" (literary grave robbing) works wonderfully, such as PD James's Death Comes to Pemberley, for example, or John Gardner's post-Fleming Bonds. There are some good things in the bubbling "fanfic" cauldron on the web, such as Cassandra Clare (web name Cassandra Claire, real name Judith Rumelt), who does great things with Lord of the Rings and Harry Potter. Try out her Very Secret Diaries. It's fanfic so it's free.

But please, let Dame Agatha, and Hercule, rest in peace. Having said that, of course I'll download it 30 seconds after it comes out in 2014.