The heaviest load ever to travel through the West will bring traffic chaos to two motorways this weekend.

The monster load, weighing 474 tonnes, will take three days to travel the 100 miles between Avonmouth and Didcot in Oxfordshire.

It is so heavy, highway chiefs have banned it from travelling along one section of the M4 because it could bring down a couple of bridges over other roads and a railway. So the huge load will travel around the Bristol ring road and across country before rejoining the motorway at Bath.

The load is a key part needed for the upgrade of a power station at Didcot, and will trundle along at less than 5mph.

The slow-moving convoy consists of a specialised load transporter of a tractor unit pulling at the front and another pushing at the back. Police vehicles will surround the load, and the entire convoy is 18ft wide, enough to take up two lanes of the motorway, 17ft high and almost 300ft long.

It sets off from Avonmouth on Sunday at 5am, and will head on to the M5 northbound.

By mid-morning, the convoy will have reached the top of the M32, where the load will head south and then along the A4174 ring road to the north and east of Bristol.

It will travel throughout Sunday along the A-roads in south Gloucestershire, ending the day at a layby at Tog Hill, off the A420 near Bath.

The load will then continue its journey on Monday at 10pm, heading along on to the M4 at Junction 18, and ending in the morning at Membury Services, on the other side of Swindon. The third stage of the journey will happen on Tuesday night, when it heads into Oxfordshire and to Didcot.

All other vehicle traffic will be halted before the M4 bridge over the River Avon near Junction 17, while the load crosses,.