Chipping Norton author Clare Mackintosh said she was “stunned” after being awarded a top crime writers' prize last night.

The former police officer beat JK Rowling’s alter ego Robert Galbraith and two-time winner Mark Billingham to win the Theakston Old Peculier Crime Novel of the Year award for her debut novel I Let You Go.

Mum-of-three Mrs Mackintosh, who worked for 12 years as a police officer, began writing full-time after leaving the force in 2011.

I Let You Go, which was selected as the winner from a longlist of 18 books and a shortlist of six, is about a woman who moves to a remote part of Wales after her child is killed in a hit-and-run accident, and the detective who is investigating the case.

Mrs Mackintosh, 39, one of the founders of Chipping Norton Literary Festival, was with Thames Valley Police for 12 years until she left to take up writing full-time.

Speaking from Harrogate in North Yorkshire, where she picked up the £3,000 prize, she said: "I’m stunned to win this award – it’s insane.

“I would not have been able to write the way I did if I had not had my career in the police force.

"I absolutely loved my time in the force but I’m happier to be a crime writer than a crime fighter.

"During my time in the force I worked in CID at St Aldate’s, as a sergeant in Chipping Norton and as an operations inspector in charge of big events like St Giles Fair.”

Mrs Mackintosh said she was thrilled to be presented with a silver tankard with her name on and her second novel, another domestic suspense thriller entitled I See You, will be published on Thursday by Little, Brown.

The author lives with husband Rob Mackintosh, son Josh, nine, and twins Evie and George, eight.

She added: "If I am disciplined I can write while the children are at school.

"I See You is a psychological thriller about a woman who finds her photo in an ad in the classifieds in a London newspaper.

"When I left the force I never imagined that writing would turn into a full-time career."

I Let You Go was one of the fastest selling titles of 2015 and became a Sunday Times bestseller and a Richard & Judy book club winner.

Praised widely for its astonishing twist, overseas rights have now sold in 30 countries.

Mrs Mackintosh added: "I first came to Harrogate as an unpublished author so to win this award tonight is a dream come true."