The new stamp costs 30p, the price of sending an air-mail letter, weighing up to 20g, to any destination in Europe.
It will be the first British stamp, aside from special-issue designs, to make any reference to geographical location, since Britain is the only country in the world identified on stamps by its monarch's head rather than its name.
According to Royal Mail officials, "non value indicator" stamps - such as those marked by "1st" or "2nd" instead of a price - have proved popular with customers since they can be used even if there is a rise in postage rates.
But although the stamp can be interchanged with standard 30p stamps to send letters to any international destination, Eurosceptic politicians have condemned its introduction as another sign of European encroachment into national life.
"The Post Office is taking a liberty with its customers, trying to sell stamps which are part of the great campaign of brainwashing the public with 'eurocreep'," said Teresa Gorman, Conservative MP for Billericay.
"It is just something else that is being slipped in so that the culture of Europe can take hold. I don't think it will wash with the public who will see it as a further assault on our independence."Reuse content