Callum Wilson struck in injury time following a corner and the goal was given on field by referee Chris Kavanagh. However, VAR Darren England recommended Kavanagh review it after spotting a foul by Pablo Felipe on Gunners goalkeeper David Raya and Kavanagh changed his mind after studying the replays.
The decision is arguably the most pivotal VAR call in Premier League history as it boosted Arsenal's chances of winning the title and denied West Ham a crucial point in their fight against relegation.
The Hammers are understood to have contacted referees' body Professional Game Match Officials (PGMO) to raise their concerns and seek explanations regarding the incident.
Webb, the chief refereeing officer at PGMO, is in absolutely no doubt, however, that it was a clear and obvious foul.
Speaking on on UK television Tuesday night (local time) Webb said: "Is it a foul on the goalkeeper? Categorically yes.
"We've said all season, including in pre-season briefings with the players, that if a goalkeeper is impeded by an opponent grabbing or holding their arms and therefore they can't do their job, they'll be penalised.
"We're not just talking about contact with goalkeepers, we're talking about a specific type of contact when the goalkeeper's arms or hands are being interfered with, stopping them doing their job.
"On the video, it's clear and it's obvious and it happens early. And, even though it's not clear and obvious to the referee because he's got a huge group of players in a penalty area and it's difficult to see, when the VAR sees this, of course, they have to get involved."
The full discussion between Kavanagh and the VAR team was played on television and England is heard advising Kavanagh to also check other possible fouls too, including the timeline in relation to the foul on Raya.
