The first of five weekend closures of the railway bridge takes place in less than a fortnight.
The closures are expected to cause congestion and delays in the area and along the diversions.
The Highways Agency says that the essential engineering work on the bridge over the Arun Valley railway line will mean the A27 has to be closed in both directions.
Cars and lorries will be diverted through Littlehampton and Bognor during the closures from 8pm on Friday nights until 6am on Monday mornings.
The work includes resurfacing and brick repairs and is scheduled for September 12 to 15, September 26 to 29, October 10 to 13, October 17 to 20 and October 31 to November 3.
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