The blaze - which is now raging across farmland the size of three football pitches - has caused c2c to cancel all services on the nearby trainline.
The fire is one of a number of grass fires which have broken out today, on the hottest day in UK history after a temperature of 40.2C was recorded at Heathrow Aiport.
Thirty fire engines and 175 firefighters from Hornchurch, Harold Hill, Wennington and Essex Fire and Rescue Service scrambled to the blaze in Pea Lane, Havering, at 11.23am.
The London Fire Brigade said: “Around three hectares of a cornfield and some scrubland is alight and smoke is blowing over the M25.
“The Brigade’s 999 Control Officers have taken more than 80 calls to the blaze.”
The M25 is still open however drivers are warned to be cautious.
c2c has cancelled all trains running between Upminster and Ockenden.
A bus replacement service is running, and Greater Anglia is accepting c2c tickets on its routes.
More to follow.