The man, in his 50s, was involved in a collision with a lorry at around 11am this morning, near the M11 junction.
He passed away at the scene despite efforts of other motorists before paramedics arrived.
Police say the road will remain closed for several hours.
No arrests have been made but investigators are appealing for witnesses to get in touch.
Anyone with dash cam footage, or who witnessed the incident and has not yet spoken to Essex Police, is asked to call the Serious Collision Investigations Unit on 101 quoting incident 376 of 16/04.
Alternatively, you can call Crimestoppers in complete confidence on 0800 555 111.
Tags: M11, M25
