A secret letter from Lord Admiral Nelson to his mistress Lady Emma Hamilton, sent just a fortnight after the birth of their love child, Horatia, has sold for £23,750 at auction.

That was nearly nine thousand pounds more than the £15,000 the letter had been expected to fetch at the auction at Christie's in London.

Each word in the two hundred word letter is now worth more than one hundred pounds.

Lord Nelson, who was born in Burnham Thorpe, north Norfolk, and Lady Hamilton were worried their clandestine correspondence was being intercepted and in the letter, dated February 12, 1801, Nelson responds to concerns raised by Emma about the fate of an earlier letter.

The couple's love child, Horatia, was secretly born on January 29 1801.

Lord Nelson and Lady Hamilton pretended Horatia was the daughter of a fictitious Admiral named Thomson and that they were the child's godparents.

In the letter Lord Nelson tells Lady Emma: 'Mrs Thomson's friend desires you will assure her of his unalterable and affectionate regard and begs she will be assured that all the world cannot either change or make him wish to change for a moment and that he is unalterably hers.'

Nelson ends by asking Emma to 'kiss my god child.'

Auctioneers Christie's said: 'Reflecting the precarious circumstances in which Nelson and Emma were forced to conduct their relationship, the letter betrays some of the insecurities that beset the couple, including the constant fear that one of their letters – full of thinly-veiled affection, with neither party well disguised by their use of the 'Thomson' alias – would fall into strange hands. From his proclamations of affection, it also seems that Emma, or 'Mrs Thomson' as she appears here, had questioned Nelson's devotion; accusations of emotional infidelity and neglect recur throughout their correspondence and saw him whipped into agonies of jealousy and despair.'

Do you have an historical artefact? Write to doug.faulkner@archant.co.uk.