The couple would eventually give birth to a daughter in 1979.
The Church would not recognise the marriage of a divorced woman with a living ex-husband as valid.
Their marriage was historic for many reasons, although their 18-year marriage was littered with infidelity.
Princess Margaret's daughter was as high as seventh in line to the throne, but now she's 24th in the line of succession.
This was a subtle attempt to separate the two love birds without causing too much drama, as Royalty feared it could harm their image if they were to get married.
In it, the Countess of Snowdon is seen soaking in a bathtub wearing nothing but the Poltimore tiara on her head; the racy image was taken in 1962 by her husband, but didn't enter the public domain until 2006, years after Margaret's death.