A broken down horsebox is causing around three miles of congestion around the Orwell Bridge.
National Highways East said in a tweet that one lane of the eastbound carriageway between junction 56 and junction 57 has been closed.
This is due to a horsebox breaking down at the Wherstead Junction earlier on Sunday morning.
It is adding around 20 minutes to motorists' journeys and has created around three miles of congestion.
Suffolk police is on the scene, and a spokesman said it will be cleared "soon".
Traffic is queueing over the bridge in the westbound lane, past the Shell garage just before the bridge.
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