Kuryu Matsuki, James Bree and Lewis Dobbin were all on target in a chaotic first half, while Kyle Walker-Peters and Tomas Soucek replied for the Hammers.

Nuno Espirito Santo’s side, relegated on the final day of last term, would have gone top with a win but instead saw their nine-match unbeaten run grind to a halt.

There were thousands of empty seats by the end at the London Stadium, with those who stayed booing the Hammers off.

For anyone who thought promotion would be achieved at a canter in east London, this was a wake-up call.

The Hammers now have a Southampton side, who were deducted four points at the start of the season, in their rear-view mirror, lying just two points behind.

Saints took the lead inside two minutes after Konstantinos Mavropanos and Divine Mukasa stood back and admired as Leo Scienza danced into the penalty area.

Scienza blasted his shot straight at Hammers goalkeeper Mads Hermansen, who made a fine point-blank save, but the rebound rolled to Matsuki who tucked it into the opposite corner.

West Ham tried to hit straight back with Walker-Peters finding Jarrod Bowen, who cut inside but dragged his shot wide.

However, the hosts were two down after 10 minutes when Casper Jander’s cross was tipped away by Hermansen as far as Bree, who without breaking stride lashed a spectacular volley into the roof of the net.

Mukasa almost made amends for his earlier error when he won the ball off Ryan Manning and jinked inside Scienza only to see his shot deflected wide.

West Ham did claw one back when former Saint Walker-Peters skipped past Jander on the edge of the box and launched a rocket into the top corner.

The Hammers were pressing incessantly for an equaliser but then promptly shot themselves in the foot as more lacklustre defending allowed Finn Azaz to shoot straight at Hermansen, with Dobbin this time snaffling the rebound.

The unlikely figure of Mavropanos almost pulled one back after the break but his rasping drive was well saved by Daniel Peretz, before fellow centre-half Max Kilman trotted forward and crashed a shot against the crossbar.

Arne Engels then took aim from the edge of the box but Peretz saved with his legs, which was the cue for scores of West Ham fans to make their way towards the exits with 10 minutes still remaining.

Those who left missed substitute Soucek curling in a consolation in stoppage time, before Peretz kept out a header from Taty Castellanos to preserve three points for Saints.