Referee Robert Madley was "200 percent wrong" to allow Juan Mata's goal to stand during Manchester United's 3-0 FA Cup fifth-round win over League One Shrewsbury Town, according to former FIFA official Mark Halsey.
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Spain playmaker Mata scored from a direct free kick in first-half stoppage time on Monday but three teammates were in an offside position several metres beyond Shrewsbury's defensive wall, obscuring goalkeeper Jayson Leutwiler's view.
The match was televised live on BT Sport and guest summariser, former World Cup final and Champions League final referee Howard Webb, said Madley was right to allow the goal to stand.
Webb's view is that there is no issue as long as the trio are back onside when the ball is struck.
Halsey, however, took the opposite view: "Without a shadow of a doubt it is 200 percent offside, whatever anyone says," Halsey told Reuters in an interview on Tuesday.
"I don't understand for the life of me how Howard Webb can say it was a good goal. He's wrong, absolutely wrong.
"I think Graham Poll has also said it was offside and I work with Keith Hackett at www.you-are-the-ref.com and we all agree it was offside," said Halsey, referring to two former refereeing colleagues.
The 54-year-old Halsey, who was on the referees' list for world governing body FIFA between 2000 and 2006, explained why the goal should not have stood.
"It's a tactic and they are seeking to gain an advantage," he said.
"Once Mata kicks that ball those three players are then interfering with an opponent...they are in the goalkeeper's eyeline.
"When they are just standing there and play is dead they are not deemed 'active' but the minute that ball becomes live and Mata puts that shot in, they then become 'active' and interfering with an opponent and with play," Halsey said.
"Being in an offside position is not an offence in itself but once that player makes a move towards the ball or challenges a defender for the ball he then becomes 'active' and is interfering with play."
Halsey, who was a Premier League official between 1999 and 2013, was equally perplexed by United manager Louis van Gaal's post-match summary.
"We have learned from the Midtjylland game," said Dutchman Van Gaal, referring to a similar incident that almost cost his team a goal in their 2-1 Europa League last-32 first-leg defeat in Denmark on Thursday.
"We think that is always offside so we have asked the referees and they are saying, 'No, you can do it'."
Halsey responded: "He must have misread their information and I was amazed the officials let the goal stand.
"The officials should have had a little conversation with each other, there was a clear communications breakdown. The goal should have been disallowed and Shrewsbury can feel very upset it wasn't."
Halsey said a similar goal would definitely be ruled out if it occurred in European competition this week.
"If it happens in the Champions League or Europa League that will be given offside because (UEFA's) elite have been told that's offside," he explained.
"It is just as well it hasn't happened in a Premier League game when it was 1-1 and someone gets a winning goal in the last minute because all hell would have broken loose," added Halsey.
"We have got to be uniformed on this and everyone's got to be on the same hymn sheet by saying that is offside."
