Double white dwarf binaries are a leading explanation of the origin of type Ia supernovae, but no system exceeding the Chandrasekhar mass limit (1.4 M⊙) has been found that will explode anywhere close to a Hubble time. Here we present the super-Chandrasekhar mass double white dwarf WDJ181058.67+311940.94 whose merger time (22.6 ± 1.0 Gyr) is of the same order as a Hubble time. The mass of the binary is large, combining to 1.555 ± 0.044 M⊙, while being located only 49 pc away. We predict that the binary will explode dynamically by means of a double detonation that will destroy both stars just before they merge, appearing as a subluminous type Ia supernova with a peak apparent magnitude of about mV = −16 (200,000 times brighter than Jupiter). The observationally derived birth rate of super-Chandrasekhar mass double white dwarfs is now at least 6.0 × 10−4 yr−1 and the observed rate of type Ia supernovae in the Milky Way from such systems is approximately 4.4 × 10−5 yr−1, whereas the predicted type Ia supernova rate in the Milky Way from all progenitor channels is about sixty times larger. Hence, WDJ181058.67+311940.94 mitigates the observed deficit of massive double white dwarfs witnessed in volume-complete populations, but further evidence is required to determine the majority progenitors of type Ia supernovae.