Anniversary celebrations for the world’s oldest high IQ society have been given an official “stamp” of approval.

Mensa, which was formed in England on October 1, 1946, celebrates its 70th anniversary on Saturday.

In the UK, Royal Mail will be joining in the celebrations by franking all mail with a special anniversary postmark.

To get the special mark, items need to be posted today so that they are sorted and franked for delivery tomorrow.

A spokesman for Royal Mail said: “Royal Mail’s postmarks are reserved for special occasions and are used to recognise significant events, support charities and highlight historical anniversaries.

“The Mensa postmark will be applied to millions of items of mail nationwide for delivery on Saturday 1 October. It will say ‘Mensa, The High IQ Society, 70th Anniversary 1946 – 2016’.”