Welsh felt that Gareth Anscombe try should not have been cancelled out by television match official at Twickenham.
Warren Gatland upset by 'terrible mistake' to disallow try as Wales lose out to England in Six Nations
Warren Gatland hit out at the "terrible mistake" not to award Wales a first-half try in their narrow Six Nations defeat against England at Twickenham.
Jonny May ran in two tries in the opening 20 minutes to send defending champions England on their way to a 12-6 victory, but Gatland's side turned in a determined performance to claw their way back into the Test.
Gareth Anscombe appeared to hand the visitors a lifeline when he crossed the line with his side trailing 12-0, and moments after May's second try, but television match official Glenn Newman controversially ruled out the score.
Anscombe appeared to exert downward pressure on the ball with an outstretched hand, but Newman disagreed, and Gatland believed the decision may have cost Wales victory.
"It looked like a try to me," Gatland said. "It is disappointing that you get that decision wrong.
"It is a pivotal moment in the game. The TMO has one big call to make and he has made a terrible mistake. At this level it is pretty disappointing. Not to be given what we thought was a legitimate try was a massive moment.
"I struggled with the wording [from the TMO]. He said that England got there first and there was no downward pressure from Wales. I saw that differently.
"There was clearly downward pressure, and at this level, in front of 82,000 people, when there is a lot at stake, you have to get those decisions right."
Asked if he will issue a formal complaint about the decision, Gatland replied: "I will just get clarity. You cannot do anything about it. It is a decision that has gone against us."
England head coach Eddie Jones - who oversaw his side's second win in as many weeks as they bid to win the championship for a third consecutive year - had no complaints with the TMO's verdict.
"I have never made a comment on a TMO," Jones said. "It was one part of the game that has done really well in rugby.
"We have a guy up there that is a referee. He has got time to make a decision, and if he can't make the right decision then what do we do? I allow him to make the decisions. He made the decision and we get on with it."
England raced out of the blocks at Twickenham as May opened the scoring with less than three minutes gone following a pinpoint kick from Owen Farrell.
May then latched on to a fine one-handed offload from Joe Launchbury midway through the opening half to double both his and England's tally. Jones' side appeared on course to run riot, but their progress was halted by a resilient Wales.
"The win was built around a lot of courage and a lot of belief in the team," Jones added. "I thought the execution of the game plan by the players was absolutely outstanding and our application of courage and effort in defence was first class.
"Following on from the win against Australia in November, it showed we can be in an arm-wrestle, we can hang in there, find a way to win and that is an important habit to have.
"You get that habit by working harder than other teams. We have to continually remember that. When you have success, it is easy to sit back and complacency sits next to you. It is important we keep doing those things and that allows you to win those arm wrestles."