Delays expected as Didcot power station transformer moved
Drivers are being warned of "severe disruptions" while a large transformer is moved from a power station in Oxfordshire to Bristol.
The 637-tonne load - which measures 294ft (89m) long - has a police escort on its journey from Didcot power station to Avonmouth Docks.
Thames Valley Police warned of disruption on the A34 from 19:00 GMT.
The convoy is taking up both lanes of the southbound carriageway, travelling at a maximum of 10mph.
A police spokesman said it would "effectively close" the southbound A34 from Didcot to the M4 Junction 13 in the section it is travelling on.
The disruptions are likely to continue over the weekend, when the load will travel along the M4 westbound.
After its arrival in Bristol, the transformer will be transported by sea to a Siemens plant in Germany.
The generator transformer is the second of three being removed from Didcot power station as part of its decommissioning programme.
The first was moved at the beginning of the month.
Didcot, a coal fired power station, was disconnected from the National Grid in March 2013 after 43 years of service.