Saka’s failure to convert would prove costly, as just two minutes later, Jarrod Bowen swept in to pull the Hammers level.
West Ham came close to a late winner, but Antonio’s header came back off the woodwork.
It is the first time that Arsenal have given up a two-goal lead in consecutive Premier League matches, making them only the fifth side to do so in the competition's history.
Arsenal remain top on 74 points, four more than Man City, but having played a match more.
The Gunners are next in action on Friday, when they host bottom side Southampton, while Man City's next Premier League match is on 26 April, when Arsenal visit the Etihad Stadium in what looks set to be a title-defining showdown.
West Ham sit 15th on 31 points, four clear of the bottom three.
United go third
In the second half, Bruno Fernandes had a shot superbly tipped on to the crossbar by Navas, before the visitors doubled their lead on 76 minutes.
Antony ran at the Forest defence before playing a clever ball into the path of Dalot, who slotted in for his first Premier League goal, and Antony's first assist in the competition.
The result moves Man Utd on to 59 points, six clear of Tottenham Hotspur in fifth.
Forest remain 18th on 27 points, in the relegation zone due to an inferior goal difference to 17th-placed Everton.
Can Liverpool boost European hopes?
The Reds are looking to end their poor away form as they seek a first win on the road in 2023
Meanwhile, Leeds are looking for a positive response following their 5-1 defeat to Crystal Palace last time out.