A thief snatched cash from a till after asking to swap some £50 notes for smaller denominations.
Police want to speak to anyone who witnessed the theft which happened just after 2pm yesterday (Sunday) in Wellingborough.
A spokesman for Northants Police said: “A man went to pay for items in Marks and Spencer, London Road.
“He asked the cashier to exchange a number of £50 notes into smaller denominations, then grabbed more than £300 worth of £10 and £20 notes, without handing over the £50 notes, and ran out of the store.
“While running from the store, he was seen to shout at a man driving an electric-blue coloured car which may have been a Corsa and then running into the Tesco car park.”
Witnesses, or anyone with information about the theft, is asked to contact Northamptonshire Police on 101 or they can call independent charity Crimestoppers anonymously on 0800 555111.