The A12 in Martlesham is backed up after a car hit the centeral reservation.
Traffic travelling northbound on the A12 near Martlesham is at a standstill after a diesel spillage closed one lane.
The closure affects the section between the A1214 Main Road and the B1438 junction, more commonly known as the Seckford roundabout.
Police have closed the road and Suffolk County Council's Highways team are on site to clean the diesel from the road surface.
A police spokesman said: "We were called shortly before 12.45pm to reports of an unknown substance on the carriageway."
The substance was later identified as diesel.
Motorists are being warned to expect delays as cars queue along the two-mile stretch of road.
One driver contacted us to say the road was at a standstill. "Nothing is moving," she said.
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