Man, 52 admits to Tesco blackmail.
Philip McHugh, 52, pleaded guilty to three charges of blackmail and two of communicating a bomb hoax targeting the retailer in July this year.
His threats, made in letters to the Hertfordshire-based supermarket chain, saw 14 stores closed as a security precaution.
McHugh, of Clitheroe, Lancashire, was remanded back into custody at Bedford prison.
Psychiatric reports will be carried out into his mental state before he is sentenced at St Albans Crown Court at the end of January.
McHugh admitted three specimen charges of blackmail in which he tried to extort pounds 1m out of the retailer in letters to the Cheshuntbased chain in June and July of this year.
He also admitted two charges of communicating a bomb hoax on July 14.
McHugh, the court heard, has no previous convictions and had been under the care of a community psychiatric nurse in the months before he sent the letter.