Police are fingerprinting anonymous letters sent to three charity shops and a newsagent, threatening to vandalise their premises if they carry on selling Christmas cards out of season.

One of the Leeds outlets of the mental health campaign Mind has temporarily withdrawn bargain cards left over from last year after its door locks were superglued, costing £100 in repairs.

Police were studying CCTV film of a hooded figure hand-delivering one of the notes to an Oxfam branch in the suburb of Headingley. The messages claim to come from the "Movement for the Containment of Xmas", purportedly trying to stem the commercial bonanza before Christmas.

The Mind branch was also phoned by a man who claimed to have carried out the attack, saying that it would happen again if cards continued to be sold "far too early". But the charities involved said that agreeing to the demand would lose them regular income. The brief notes, all delivered by hand, say: "This is a very polite but very serious reminder not to display Xmas cards until 1st Nov. We will put super glue into your locks if you do. Peace and goodwill.'"

Oxfam manager Clive Barker said the letter had arrived on Monday evening. He said: "It is very odd. Every morning I wonder if I am going to be able to open up, or will the lock be glued up? The cards are important for our fundraising. Like the rest of the high street there are Christmas cards on display as early tasters. Four shops have been targeted and I just hope that nothing comes of it. The police say they are treating it very seriously and have taken the letter for fingerprints."

The Mind shop has taken cards off display, but the third target, the animal charity PDSA, will start selling new stock shortly.