The HGV lorry jack-knifed near Gatwick Airport and blocked the road, with one lane of the road closed by police while the incident was dealt with.
Access to the A23 was closed from the Longbridge roundabout near the airport for several hours while the lorry was recovered.
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Royal Mail say that their driver did not suffer any serious injuries and that they have launched an investigation into the incident.
A Royal Mail spokesman said: “We can confirm a Royal Mail van was involved in an incident on the A23 on Saturday.
“We are pleased to confirm that following the incident, the driver of the Royal Mail vehicle has not suffered any serious injuries.
“We have launched an investigation into what happened and are helping the police with their enquiries.”
The Royal Mail lorry was left strewn across the A23 at around 7pm yesterday evening following the crash.
Parts of the central reservation appear to have been damaged in the crash.
The road has since re-opened.
Sussex Police have been approached for comment.