Butt sparked outrage in the England dressing room after suggesting the players took money to lose the third one-day international at the Oval last Friday.
Despite calls for them to withdraw from the series, England reluctantly played out the remaining two games, eventually clinching a 3-2 win at the Rose Bowl on Wednesday.
But now, the England and Wales Cricket Board has joined forces with the Professional Cricketers Association to draft a letter to Butt threatening legal action if no apology is made.
A statement confirmed: “The letter seeks a full and unreserved apology. Mr Butt has been advised that if a satisfactory response is not received, legal proceedings will be commenced against him without further notice.”
Meanwhile, police investigations are continuing into allegations that Test captain Salman Butt and fast bowlers Mohammad Amir and Mohammad Asif were part of a spot-fixing plot during the fourth Test at Headingley. All three have been suspended by the International Cricket Council.