The Lloyds Pharmacy at Newbattle Medical Centre in Dalkeith was targeted when it was busy with customers. The thief escaped with the charity box thought to have been half-full with donations.
Police have launched an investigation into the “galling” theft and are combing CCTV footage from the area.
Neil Griffiths, a spokesman for the Royal British Legion Scotland, said the theft of the collection tin would have a direct impact on cash available to the relatives of fallen servicemen.
“We lose about 60 collection tins a year across the whole of Scotland,” said Mr Griffiths. “It’s always the safest places that they seem to be stolen from like banks and police stations.
“It makes my blood boil and if possible I would like to get hold of the culprits and have them spend a day working with me so I could show them what we do with this money.
“They are stealing from the wallets of really needy people – about half of these are widows.
“It makes a massive difference to the needy victims of recent conflicts especially widows.”
Asked about the potential sum stolen, he added: “People would be surprised to learn how much would be in a tin. It’s not a number we give out but we can say that money would have made a real difference to somebody.
“Theft of collection tins is a small scale problem but nevertheless it’s infuriating for all that.”
Sgt Scott Meechan, a community policeman for the Dalkeith area, said: “This is a particularly galling theft as the money has been taken from a very worthy cause, with Remembrance Day coming up.
“It’s a heartless crime and while it’s never nice that any money is stolen from charity it’s particularly galling given the proximity of the event.
“We would appeal to anyone who may have been in the area, and has any information regarding this, to get in touch.”
Andrew Beattie, pharmacy manager at the Lloyds branch when the theft happened at around 4.30pm last Wednesday, said: “It’s upsetting to think that donated money for this charity has been stolen. Since the theft of the collection box, customers have continued to make donations.”
Anyone with any information should contact Lothian and Borders Police on 0131-311 3131.