Cover shows 1st class stamp weeks before issue.
It appears some of the Doctor Who stamps have been released four weeks early, after a cover bearing the 1st class Matt Smith design was posted into a newspaper competition.
The stamp, which seems to have been received on February 28, is not due for release until March 26.
The cover has a Glasgow Mail Centre postmark, but the date of this is unclear.
A Royal Mail spokeswoman said: 'Regrettably, it does appear that a stamp has been sold at a Post Office in advance of the official launch date and used on a mail item. Post Office branches are being reminded that they should adhere to the first day of issue policy and not offer the stamps for purchase to their customers before then.'
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