The electrical equipment is so big that hedges along the southern carriageway have had to be trimmed ahead of the mammoth task, which will lead to delays along the London orbital motorway.
Those using the southern stretch of the M25 will risk getting stuck in delays as the house-sized transformer moves to West Weybridge substation in Surrey from Tilbury Docks in Essex.
It will travel along the M25 from junction 2 to junction 11 at the end of next week and could affect drivers heading back to west London from Surrey, Sussex or Kent.
Letters have been sent out by National Grid to nearby residents who may be affected.
The electrical equipment will be moved at 7pm on Friday August 17 and will be transported to the Surrey substation by 4pm on Sunday August 19, Kent Live reports.
National Grid says the journey is expected to take up to 45 hours.
For most of its journey, the transporter will be on a specialist vehicle which is more than 68 metres long and 5.3 metres wide, travelling at 12mph.
But "to keep disruption to a minimum" there will be a 24 hour stop at Clacket Lane Services to ensure the delivery can be made on Sunday when there is less traffic on the road.
What's its journey?
- It will leave Tilbury Docks via the A109 and will travel north until it meets the A13
- It will then go along the A13 and onto the A1306.
- It will then continue up until the junction of A282 towards the Queen Elizabeth II bridge on the Dartford Crossing
- Once over the bridge, it will exit at junction 1A and go along the A206, B255 and A2 before rejoining the M25 at junction 2
- The transformer will exit the M25 at junction 11 and continue on local roads until it reaches its destination
People planning to use any of the above routes in the allotted times have been urged to re-think their journeys or allow extra time.