London Buses route 423 is a Transport for London contracted bus route in London, England. Running between Hounslow and Heathrow Terminal 5, it is operated by London United.
Route 423 commenced operation on 22 March 2008 between Hounslow Bus Station and Heathrow Terminal 5 via Hounslow High Street - Douglas Road - Grove Road - Staines Road - Barrack Road - Corporation Avenue - Martindale Road - Beavers Lane - Salisbury Road - Green Lane - The Causeway - Great South-West Road - Dick Turpin Way - Fagg's Road - Hatton Road South - Hatton Cross - Southern Perimeter Road - Hatton Cross Bus Station - Southern Perimeter Road - Hatton Cross - Envoy Avenue - Envoy Roundabout - Envoy Avenue - Eastern Perimeter Road - Northern Perimeter Road - Hatton Road North - Bath Road - Colnbrook By-Pass - Bath Road - Bus Bridge - Western Perimeter Road - Western Perimeter Road Slip Road - Windsock Way - Wallis Road. The route was introduced to coincide with the opening of Terminal 5 at Heathrow Airport. It was an extension of route H23, that operated from Hounslow Bus Station to Hatton Cross Station. The route was initially operated by London United using Plaxton Pointer 2 bodied Dennis Dart SLFs.
On 12 February 2011, the Plaxton Pointer 2 bodied Dennis Dart SLFs were replaced by Alexander Dennis Enviro 200 Darts.
On 3 March 2011, the route was retained by London United.
On 3 March 2018, the route was retained by London United.
Route 423 operates via these primary locations:
- Hounslow Bus Station
- Hounslow Heath
- Beavers Farm Estate
- Hatton Cross Station
- Harlington Corner
- Heathrow Airport North
- Heathrow Terminal 5