Andrew Balding's charge had lifted the race 12 months ago and was sent off the 2-1 favourite to back it up.
The result looked unlikely a furlong out as Sir Michael Stoute's Zaaki was still on the bridle while Silvestre de Sousa was hard at work.
As Zaaki looked set to extend his winning streak, Beat The Bank gamely found more for pressure and had his nose down where it mattered most.
However, it soon became apparent all was not well and De Sousa dismounted quickly with Beat The Bank eventually leaving the track in a horse ambulance.
Having been assessed by the Ascot vets, Beat The Bank was found to have sustained a complicated fracture and had to be euthanized.
Balding tweeted: "Tragically Beat The Bank suffered a fatal injury after winning the Summer Mile for the second year running.
"He was such a brave horse and everyone at Kingsclere and King Power (Racing, owner) are heartbroken. We will remember him forever."
Marvellous win for Cox
Wokingham runner-up Tis Marvellous justified 9/2 favouritism with a clear-cut win in the Betfred Heritage Handicap at Ascot.
Clive Cox's charge found only Cape Byron too good at the Royal meeting and proved a cut above his rivals here.
Adam Kirby produced the market leader to lead at the furlong marker and he quickly put the race to bed, bounding two-and-a-half lengths clear of nearest pursuer Open Wide (12/1).
Front-running Harome (16/1) took third ahead of Embour (14/1).
