Austrian Arnautovic, who had last found the net on Jan 2, bagged the opener after Yan Valery misdirected a 16th-minute pass to Mark Noble.

The midfielder slipped the ball through for Arnautovic, who produced a typically clinical finish past Fraser Forster.

Southampton’s keeper was back between the posts for the first time since December, 2017 and will be disappointed with his failure to clear danger ahead of Arnautovic’s second on 69 minutes.

Forster only succeeded in punching the ball at the striker who headed home from close range for his fifth goal against the Saints in as many games.

Fredericks completed the rout, firing home after a clever one-two with Jack Wilshere three minutes later.

Before the Hammers’ second-half goal burst, Southampton had looked the more likely to score, with the lively Nathan Redmond, who had replaced Danny Ings at the interval, forcing a fine save from Lukasz Fabianski.

But the visitors always looked vulnerable to the counter-attack and Arnautovic went close to securing his hat-trick when he curled a late shot wide.