In the letter, Cromwell discusses intimate family affairs such as his son’s marriage and the fate of his two younger daughters.
The document will be sold by auction house Lyon & Turnbull on Wednesday September 28.
Cromwell led the Parliamentarian forces against the Royalists in the English Civil War and oversaw the execution of King Charles I in January 1649.
The letter is addressed to Cromwell’s friend, Richard Norton, who served in the parliamentary army, a month before Cromwell led English troops to a bloody victory over the Scots and Royalists at the Battle of Preston.
Cromwell requests that Norton communicate his wishes to his son’s future father-in-law, Richard Maijor.
Cromwell writes: “I desire you to carrie this businesse with all privacie, I beseech you to doe soe as you love mee, let mee entreat you not to loose a day herein, that I may know Mr Maiors minde… I much rely upon you.”
Bidding for the letter will begin at £5,000 and is part of Lyon & Turnbull’s rare books, maps, manuscripts and photographs sale on Wednesday.
Also featured is an album of 120 letters by a range of Victorian figures including: Charles Dickens, William Thackeray, Robert Browning and William Holman Hunt.
One highlight in the collection is a letter by geologist Charles Lyall to the Scottish Lord Advocate mentioning a “geological appointment” with Sir John Lubbock and Charles Darwin.
Dominic Somerville-Brown, Lyon & Turnbull’s rare books, maps, manuscripts and photographs specialist, said: “We anticipate international interest in this particular auction, given the significance of these exceptional items – including Cromwell’s letter – and the two albums, which have been in the Kinnear family for generations.
“These important documents and images capture the personalities and times of a range of hugely important historical figures.
“It’s always the human stories and personal touches which capture the public’s imagination.”