The forward scored one in both halves before defender Ryan Fredericks sealed the win as the Hammers made it back-to-back wins, having beaten London rival Tottenham Hotspur last time out.

Despite being without a victory in four starts, Southampton is mathematically safe from the drop.

Arnautovic struck first in the 16th minute when Yan Valery lost the ball in a dangerous area, allowing Mark Noble to pick up possession and feed the Austrian for a cool left-foot finish.

West Ham put the result beyond doubt 20 minutes from time when Arthur Masuaku surged past James Ward-Prowse. His cross was pushed out by Forster, and a fortuitous deflection off Arnautovic allowed the forward to head in from close range.

Three minutes later, Fredericks scored his first league goal in more than five years, playing a neat one-two with sub Jack Wilshere before powering into the box and beating Forster with a low strike into the bottom corner.