<p>Blue phosphorescent organic light-emitting diodes (PHOLEDs) are highly efficient, although their short operational lifetimes have limited their commercial acceptance in displays and lighting. A high density of energetic triplet excitons accumulates in the emission layer that eventually annihilate, leading to molecular degradation. Here we report long operational lifetime, efficient and deep blue tandem PHOLEDs using the polariton-enhanced Purcell (PEP) effect at both the diode cathode and anode. The Pt- and Ir-complex-based bottom-emitting tandem PEP-PHOLEDs show a tenfold increase in lifetime and colour saturation compared with their analogous single-junction PHOLEDs. Particularly, the Pt-complex-based tandem PEP-PHOLED on flat glass substrates has lifetimes of LT90 = 830 ± 30 h and 1,800 ± 100 h (LT90 = the time to reach 90% of the initial luminance of 500 cd m⁻²), and forward-viewing external quantum efficiencies of 36.8 ± 0.5% or 56.0 ± 1.0% without or with substrate light extraction, respectively. The lifetime enhancement is equivalent to 250 times that of conventional single-junction Ir-based PHOLED lifetimes with similar blue chromaticity coordinates. The tandem PEP-PHOLED has chromaticity coordinates of CIE<i>xy</i> = (0.14, 0.12) and a marginally perceptible colour shift with viewing angle up to ±75°. The double-sided PEP effect is readily integrated with other established lifetime-extending technologies, allowing for a multiplicative increase in lifetime. To our knowledge, this is the first demonstration of a deep blue PHOLED that shows stability comparable to green PHOLEDs, accelerating the use of the deep blue phosphorescent emitters in power-efficient displays and lighting.</p>