Under the new rules – which come into force today – motorists in England, Scotland and Wales can display the Union flag, Cross of St George, Saltire or Red Dragon of Wales.
Geoff Hoon said: "The display of our national flags is a healthy expression of the pride we have in our country.
"It is right that motorists are now able to celebrate this pride by displaying flags on their number plates and I look forward to seeing our national symbols on cars around the country."
The flags can be displayed on the left hand side of the number plate.