Two people have died in separate crashes on the M4 overnight.
The first happened at about 01:35 BST when a car overturned between Bath and Chippenham, killing one man and leaving another with life-threatening injuries.
Two lorries, a car and a van then crashed in the same area between junctions 17 and 18. One man died and two others were injured.
Leaking propane gas canisters on one of the vehicles had to be made safe and the westbound carriageway remains shut.
The eastbound carriageway opened shortly before 13:00 BST.
Wiltshire Police said the westbound side was likely to remain closed for the rest of the day.
Separately, the motorway is also closed westbound from junction 16 because of another crash involving a lorry and a van.
Supt Phil Staynings said: "This is a complex incident that put huge pressure on all of the emergency services this morning and will continue to take a considerable amount of time to investigate thoroughly.
"We appreciate the public's patience on this regarding the closure of the M4 and we are working closely with Highways England to ensure we can have the road back open when it is appropriate to do so."
People are being asked to avoid the area where possible due to increased volumes of traffic on surrounding roads.