The appearance of the Queen's head on postage stamps is to be guaranteed by law in a deal struck between the Government and Buckingham Palace to protect the tradition.
Amendments will be put forward by Ministers this week to close a loophole that could have allowed a foreign buyer of the Royal Mail to remove the monarch's image.
The change was confirmed by Business Minister Ed Davey, who said his department and royal officials had "worked extremely well together to prepare for this initiative".
While the legislation paving the way for a sell-off contains a clause giving the Queen a veto over any use of her image on stamps or other products, it does not insist that her head be shown.
Officials had justified the lack of a specific protection by saying it would be "commercial suicide" for any operator to remove the royal head - and that it had not be requested by the Palace.
German and Dutch operators are expected to be leading bidders in the sell-off and Labour accused the coalition of watering down protection to attract maximum interest.