A mental health nurse has won the Costa Book of the Year for his debut novel, upsetting the odds to beat favourite Kate Atkinson, with the chair of the judges saying it could be the start of a glittering literary career.

Nathan Filer was awarded the 2013 prize and a cheque for £30,000 for The Shock of the Fall at the awards ceremony in central London. It caps a big week for the 33-year-old who got married on Saturday and was wearing the same suit to the ceremony.

He said: “It feels very nice indeed. I'm delighted. I wrote this book because I wanted to share it, not for myself. I always wanted people to read it and winning this prize means I know this will happen more and more.” His new wife Emily revealed she had been “100 per cent sure” Filer would not win the prize.

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The story of a teenager's descent into mental illness was bought by HarperCollins for a “substantial” six-figure sum after 11 publishers vied for the work.

Filer still works shifts as a registered mental health nurse and will continue to do so, and he also lectures in creative writing. “I've written since I was a little boy,” he said. “But there is something to be said to having more to your life than writing. The other work I do has always been important to me.”

Rose Tremain, the chair of the judges, said: “For a first novel it is astonishingly sure footed,” adding it was an “extraordinary achievement. This book stood out in an exceptional way”.

“He will come into the public consciousness. The world of books is extremely crowded. Books have a short shelf life and are easily forgotten and this is one of the reasons why literary prizes are important.”

This is the first time a debut novel has won the overall prize since 2006 when Stef Penney won for The Tenderness of Wolves. Hilary Mantel won The Costa Book of the Year in 2012 for Bring Up the Bodies.

Each year five category winners are announced and then one book is chosen as the book of the year.

“The prize will change this person's life,” Ms Tremain said. “It's always a risk that it could be a one hit wonder… If I had to bet on it, there will be more books than this one, and I really hope there are.”

Mr Filer said: “Of course pressure comes with this, but I'm not going to sit and bemoan a lovely thing that's just happened.” He is working on new projects but wouldn't reveal what they were.

The novel took three years to write often late into the night following his day job. Ms Tremain said: “It is not just about schizophrenia, it's about grief a subject we all have experience of. Its grief analysed and treated without sentimentality. It's an astonishing thing.”

Filer beat Atkinson who had been installed as favourite by the bookies for her book Life After Life, a day after she had won the prize for literature at the South Bank Sky Arts Awards.

She won the book of the year award for her debut Behind the Scenes at the Museum almost two decades ago, and she would have been the first woman to win the main prize twice.

Filer also beat Lucy Hughes-Hallett who wrote The Pike a lauded biography of Italian poet, politician and fascist Gabriele D'Annunzio which won the Samuel Johnson Prize.

He worked for 10 years on psychiatric wards in Bristol.

Michael Symmons Roberts won the poetry prize for the second time with his sixth collection Drysalter. He had already won the £10,000 Forward Prize.

The children's book was won by political cartoonist Chris Riddell for Goth Girl and The Ghost of a Mouse. All the winners received £5,000.

The Costas area only open to writers from the UK and Ireland and celebrate the “enjoyability” of books.