Like many identical twins, Helen and Morna Mulgray have always done everything together.

They have always shared the same bedroom, worn identical clothes and enjoyed the same books and television programmes. For three decades they had the same job – though in different schools – before retiring the same year.

But at the age of 68, the Mulgray twins, who have never been apart for longer than two weeks, created a minor piece of history yesterday when their first novel went on sale in bookshops across Britain.

No Suspicious Circumstances, a crime novel set in Edinburgh and featuring an intrepid female investigator from HM Revenue & Customs, is believed to be the first novel published in English by identical twins. The authors, from Joppa, outside Edinburgh, are described on the cover simply as “The Mulgray Twins”.

Started more than 14 years ago, it is the result of thousands of hours of painstaking writing and rewriting. Each of its 86,000 words was a joint endeavour.

“We have both spent 31 years as English teachers so have always loved writing,” said Morna yesterday. Helen added: “We used to do it in the holidays, but back then it was just a hobby. When we retired we had the time to do it seriously.”

Their writing style is so similar that judges in literary competitions have in the past mistaken them for the same person – and accused them of submitting too many entries.

They claim that they always know what the other is about to say. When in company they finish each other’s sentences; when alone, however, they rarely bother.

Morna, who is the elder by ten minutes, said: “We both sit at the laptop, and whoever happens to be closest to the computer or feels like it does the typing. The ideas and the vocabulary bounce off each other.”

Helen added: “Occasionally there are small disagreements over the text but . . .” Her sister finishes: “It rarely happens.”

No Suspicious Circumstances, which is published by Allison & Busby, follows the trail of DJ Smith, an undercover Customs officer, as she investigates a heroin smuggling ring, and her Persian cat, Gorgonzola, who is talented at sniffing out drugs.

The novel has won plaudits for its unusual style and has been praised by one reviewer as “refreshingly unique”.

While terms with Allison & Busby have yet to be agreed, the book is expected to be followed by at least two more, called Under Suspicion and Most Suspicious Circumstances. A fourth is being considered.

Both former teachers at secondary schools near Edinburgh, the twins first tried their hand at a romantic novel in the 1980s but were told that their effort contained too many jokes and was not formulaic enough.

It took them five years and more than 20 rejections before they found a publisher.

Neither has married or even had serious boyfriends.

Morna says: “What we have is unique. Once you have found your perfect companion, it’s difficult to get anybody else to match up.”

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