UPDATE: The Southwick Tunnel has reopened following a crash which caused long delays on the A27 this morning.
Police reported two vehicles collided on the westbound carriageway.
Two people are said to be injured but their injuries are not thought to be serious.
The Southwick Tunnel was initially closed in both directions. The eastbound carriageway was quickly reopened, while the westbound stayed closed until mid-morning.
Drivers are advised to seek alternative routes.
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