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.

British Royal Mail will be joining in the celebrations by franking all mail on Saturday with a special anniversary postmark.

British Royal Mail special postmark celebrating Mensa's 70th anniversary October 1, 2016

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

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’.”