Morrisons is recalling its "Traditional Chicken and Mushroom Pie" from sale because a small number of packs do not contain that pie, but a fish pie.
The fish pie contains fish and mustard which are not mentioned on the label.
The Food Standards Agency says the supermarket's pies are therefore a possible health risk for anyone with an allergy to mustard and/or fish.
It says that those bought in error should be returned to the store from where they were bought.
The product at the heart of the mix-up is the Morrisons "Traditional Chicken & Mushroom Pie" in the 400g packs, with a use-by date of 4 December 2017.
Morrisons, which is the UK's fourth-biggest supermarket chain, has apologised for the error.
It is also contacting allergy support organisations, which will tell their members about the recall.
Notices explaining why the pies are being withdrawn are being placed in stores.
No other Morrisons products are known to be affected.