Sotheby's have described it as one of the finest collections of British stamps to come on to the market in the past 25 years. The auction house said it would need three days to sell off 2,185 lots.
The collection is also unusual because there are very few stamp collectors who are women.
Lady Mairi started collecting stamps at an early age; later in life she formed important collections of Cook Islands, the Falkland Islands and Hong Kong.
Great Britain became the focus of her collecting activities.
In the collection are fine examples of the penny black and two-pence blue.