"This is a company-wide policy across the business which covers flags and stickers across trollies, bikes, vans and lorries".
A south London housing estate has been adorned with more than 300 England flags in a patriotic display hoped to propel the national side to World Cup glory in Russian Federation.
Royal Mail said 25,000 backroom staff not in the public view would be allowed to display flags.
TUV MLA Jim Allister - who a day earlier decried the rainbow-coloured Northern Irish lorry as "political bandwagon" - said the contrast between the restrictions on the England flag and promotion of the gay pride one are "pretty telling".
As the World Cup gets underway, football fans across England will be attaching their St George's Crosses to their homes and cars to show support. Royal Mail drivers will be angered'.
PAEngland flags being proudly flown in south London
Theresa May's official spokesman said: "The Royal Mail's policies are a matter for them".
The tournament begins on Thursday, with Russian Federation v Saudi Arabia kicking off at 16:00 BST.
On dangers flags can pose to the public, the company said they can "pose a hazard to other road users if they are lost when the vehicle is moving".
Local resident Geraldine Howard said her patriotic act of erecting a flag a few years ago had been followed by a neighbour, and "snowballed" from there, after residents were asked by letter if they were happy with the new decor.
"There will be flags on display in Royal Mail offices and live radio commentary playing on PA systems during matches".