The never-before-seen Valentine’s Day card was sent in February 2009 to Blake Fielder-Civil while he was serving time at HMP Edmunds Hill, now HMP Highpoint North, in Suffolk.
Fielder-Civil was serving a 27-month sentence for assault and perverting the course of justice, and was later recalled to prison after failing a court-ordered drug test.
Winehouse wrote the letter while on holiday in the Caribbean, after Mr Fielder-Civil had filed for divorce.
Amy Winehouse and Mark Ronson on stage during the BRIT Awards 2008 (Image: Yui Mok/PA)
The card reads: “My husband, my Blake.
“Virtue is violent, and violets are blue.
“Blue – bruised is my heart since its beatings for you.
“I love you, handsome, your Valentine four years strong.
“Your Amy.”
The item, which included an envelope addressed to Mr Fielder-Civil in prison and stamped from St Lucia, was expected to fetch between £2,000 and £4,000.
It was sold by Ewbank’s in Guildford on August 26.
Ms Winehouse died on July 23, 2011, at the age of 27.