Match ends, West Ham United 3, Manchester United 1.
Manchester United recorded their worst league start in 29 years as Jose Mourinho's side were beaten by West Ham at London Stadium.
They have 10 points from seven games - the same as in 2013-14 under David Moyes - but inferior goal difference means this is their worst top-flight start since they picked up seven points in 1989-90.
The Hammers, with only one win in the preceding 19 top-flight meetings, were ahead after just five minutes through club-record signing Felipe Anderson's clever back-heeled finish from Pablo Zabaleta's cross - despite both players appearing to be offside.
And the hosts doubled their advantage on the brink of half-time in fortunate fashion, as Andriy Yarmolenko's shot cannoned in off Victor Lindelof and beyond the wrong-footed David de Gea.
Marcus Rashford responded for the largely pedestrian visitors on 71 minutes, but Marko Arnautovic restored the Hammers' two-goal lead less than three minutes later as Mark Noble's pass sliced through United's unconvincing back-line.
Manchester United proved no match for a lively West Ham in the opening stages, but the visitors did go close to equalising when Romelu Lukaku's header struck the post midway through the first half.
Their appeared little response to their midweek Carabao Cup exit to Championship side Derby, however, and the Hammers - who had failed to score in their previous three home games - enjoyed plenty of chances.
Marouane Fellaini forced a magnificent save from goalkeeper Lukasz Fabianski as the visitors chased a route back into the contest, before Rashford's sublime front-post flick offered a lifeline.
But Arnautovic's fourth goal of the season signalled West Ham are beginning to progress under Manuel Pellegrini, while problems continue to mount for Manchester United and manager Mourinho.
In a week that saw them dumped out at the third-round stage of the Carabao Cup by lower-league opposition, the talk surrounding Manchester United has focused as much on off-field troubles as it has on actual results.
Mourinho's relationship with £89m record signing Paul Pogba appeared strained once again on Wednesday, as a tense exchange between the two was captured by cameras at United's training ground.
And, following the announcement that Pogba would not captain his side again, after criticising their approach to the home draw with Wolves last weekend, Mourinho reiterated his stance before this game, stating: "No player is bigger than the club."
Although French World Cup winner Pogba did start against West Ham, Alexis Sanchez - not thought to be injured - was left out of the squad entirely, along with Jesse Lingard, as United stumbled to defeat in east London.
The visitors appeared uninterested as Pellegrini's men exposed a shaky United defence, initially consisting of Scott McTominay, Chris Smalling and Lindelof, and United have now kept just one clean sheet in seven games having begun their last campaign with six shut-outs in the same period.
Pogba was brought off in the second half, having looked unable to influence the game, along with Anthony Martial, who failed to take the opportunity to impress as Mourinho's side struggled in attack.
Rather than receive a response from his players, Mourinho is now faced with bigger questions as his side drop a place to eighth in the table - after playing a game more than their rivals.
More to follow.
Match ends, West Ham United 3, Manchester United 1.
Second Half ends, West Ham United 3, Manchester United 1.
Marouane Fellaini (Manchester United) wins a free kick on the left wing.
Foul by Pedro Obiang (West Ham United).
Fred (Manchester United) wins a free kick in the defensive half.
Foul by Robert Snodgrass (West Ham United).
Substitution, West Ham United. Grady Diangana replaces Felipe Anderson.
Foul by Scott McTominay (Manchester United).
Felipe Anderson (West Ham United) wins a free kick in the attacking half.
Attempt blocked. Marcus Rashford (Manchester United) right footed shot from outside the box is blocked.
Fred (Manchester United) wins a free kick in the attacking half.
Foul by Robert Snodgrass (West Ham United).
Offside, Manchester United. Marouane Fellaini tries a through ball, but Romelu Lukaku is caught offside.
Fred (Manchester United) wins a free kick on the right wing.
Foul by Pedro Obiang (West Ham United).
Substitution, West Ham United. Michail Antonio replaces Marko Arnautovic.
Offside, Manchester United. Scott McTominay tries a through ball, but Ashley Young is caught offside.
Corner, Manchester United. Conceded by Lukasz Fabianski.
Attempt saved. Scott McTominay (Manchester United) right footed shot from outside the box is saved in the bottom right corner. Assisted by Fred.
Foul by Felipe Anderson (West Ham United).
Ashley Young (Manchester United) wins a free kick in the defensive half.
Corner, Manchester United. Conceded by Arthur Masuaku.
Mark Noble (West Ham United) wins a free kick in the defensive half.
Foul by Marouane Fellaini (Manchester United).
Ashley Young (Manchester United) is shown the yellow card for a bad foul.
Felipe Anderson (West Ham United) wins a free kick on the left wing.
Foul by Ashley Young (Manchester United).
Goal! West Ham United 3, Manchester United 1. Marko Arnautovic (West Ham United) right footed shot from the centre of the box to the centre of the goal. Assisted by Mark Noble with a through ball.
Corner, West Ham United. Conceded by Chris Smalling.
Substitution, West Ham United. Robert Snodgrass replaces Andriy Yarmolenko.
Goal! West Ham United 2, Manchester United 1. Marcus Rashford (Manchester United) right footed shot from the centre of the box to the bottom left corner. Assisted by Luke Shaw with a cross following a corner.
Substitution, Manchester United. Juan Mata replaces Anthony Martial.
Substitution, Manchester United. Fred replaces Paul Pogba.
Corner, Manchester United. Conceded by Pablo Zabaleta.
Marcus Rashford (Manchester United) wins a free kick on the right wing.
Foul by Arthur Masuaku (West Ham United).
Corner, Manchester United. Conceded by Fabián Balbuena.
Corner, Manchester United. Conceded by Fabián Balbuena.
Attempt saved. Marouane Fellaini (Manchester United) header from the centre of the box is saved in the bottom left corner. Assisted by Ashley Young with a cross.
Delay over. They are ready to continue.