The association between the two doyennes of British theatre, television and cinema stretches back to the 1950s, long before they became national treasures.
The pair appeared together in films such films as A Room With A View, Tea With Mussolini, Ladies in Lavender  and the Best Exotic Marigold Hotel, as well as numerous stage productions
The 89-year-old Dench became emotional on stage at the Cheltenham Literature Festival on Saturday when asked about the death of the Downton Abbey star and her As Time Goes By cast mate Barbara Leigh-Hunt.
Dench was asked what she meant when she compared grief to petrol.
"I suppose because the energy that's created by grief," she said before breaking down.
Dench and Smith first met as young actresses performing at the Old Vic theatre in London and Smith had previously hailed her friend's loyalty at a tribute event for the Mrs Brown star back in 2002.
"What I remember mostly about that time, it was the beginning of a friendship, and I remember laughter more than anything in the world," Smith said.
"Judi's the most tremendous friend. She's been a huge support and hugely loyal."
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