The day Britain leaves the European Union will not be an "important anniversary", Royal Mail has said in its first explanation of why commemorative stamps will not be produced.

In a letter seen by the Telegraph, a Royal Mail executive told MPs that it printed 13 sets of "special stamps" each year, but these only tended to "mark important anniversaries and activities that reflect the UK's heritage and contribution to the world".

The national postal service also said that Brexit stamps would damage its "strict political neutrality" during elections and referendums, adding it does not get involved in "political matters".

Yet critics immediately pointed to the fact that in 1973, when Britain joined the European Economic Community, Royal Mail produced stamps showing a Union-Jack emblazoned UK jigsaw piece slotting into a bigger European puzzle.

Royal Mail Director of Public Affairs and Policy David Gold said in the letter: "I am not aware of the thinking or decision-making that led to the stamps issued in 1973 by the General Post Office."