Mail is believed to have been been stolen from at least 18 boxes across Oxfordshire, Berkshire, and Wiltshire.
Royal Mail managers discovered yesterday that boxes in Manor Road, Wantage, the nearby villages of Kingston Lisle and Letcombe Regis, and two boxes in Shrivenham, near Faringdon, had been targeted.
Royal Mail spokesman Floyd Jebson said the thefts had all been carried out yesterday morning (August 27).
He added: "It looks as if the boxes had been broken in to by someone using a sledgehammer.
"The same method has been used at each box."
Anyone who fears their mail has been stolen should call Royal Mail customer services on 08457 740740.
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