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"Butt has been advised that if a satisfactory response is not received, legal proceedings will be commenced against him without further notice," an ECB statement said.
The threat follows an interview given by Butt to a Pakistan television channel after England lost the third one-day international against Pakistan at the Oval last Friday.
Butt said there had been "loud and clear talk in bookies' circles that some English players were paid enormous amounts of money to lose (Friday's) match".
In a BBC radio interview before Monday's fourth match at Lord's, Butt said he had been merely repeating information he had received from bookmakers.
His comments came after the International Cricket Council said it was launching an inquiry into Pakistan's scoring pattern at the Oval after receiving information from a British newspaper.
Thursday's ECB statement said a letter from the board and the Professional Cricketers' Association had been sent to Butt.
"The letter seeks a full and unreserved apology," the statement said.
Pakistan's acrimonious tour of England and Wales concluded on Wednesday when England clinched the five-match 50 overs series at the Rose Bowl in Southampton.
Pakistan Test captain Salman Butt and his team-mates Mohammad Amir and Mohammad Asif had been suspended after an investigation into a newspaper report that they had arranged for no-balls to be deliberately delivered in the fourth Test at Lord's.
