Fifty-six page manuscript, kept privately until now, goes up for sale in April with auctioneer expecting seven-figure sale.

A rare notebook of Alan Turing’s which dates back to his work breaking the Enigma code is expected to sell at auction for at least £1m.

The 56-page manuscript sees the codebreaker working on the “foundations of mathematical notation and computer science”, said Bonhams, which will auction the notebook in New York on 13 April. It dates from Turing’s days in Bletchley Park in 1942, and according to Bonhams’ specialist Cassandra Hatton, “its mathematical content gives an extraordinary insight into the working mind of one of the greatest luminaries of the 20th century”.