Leicester boss Craig Shakespeare said the referee "guessed" over the penalty decision that led to their defeat at Atletico Madrid in the first leg of their Champions League quarter-final.
Jonas Eriksson awarded a spot-kick for a foul on Antoine Griezmann, with replays showing the offence took place outside the box.
Griezmann scored the penalty to give Atletico a 1-0 victory in Spain.
"It is a really disappointing decision by the referee," said Shakespeare.
"It's a key moment in the game. He has to get that one right, he can't guess on those things. It's a definite free-kick but it's out of the box.
"It's the key decisions you want correct."
'It has ruined our game plan'
Goalkeeper Kasper Schmeichel was equally adamant that the decision was the incorrect one.
"It was plain and obvious to see. It has ruined our gameplan," he told BT Sport.
"It is a decision that is tough to take when it is so clear and obvious. We should have had something from this but we have to accept it.
"We made a challenge outside of the box. They might have scored from that free-kick but obviously there is a much better chance of scoring from the penalty - but it was never a penalty."
'A diabolical decision'
Former Manchester United, Everton and England defender Phil Neville on BBC Radio 5 live:
"It was an outstanding result. Craig Shakespeare would have taken that before tonight.
"Leicester have defended really well and limited Atletico Madrid to shots from distance. It was just a horrendous penalty decision that has cost them the game.
"We have no monitor and no television replays and I knew straight away that Marc Albrighton's challenge was outside the box. We must be about 80 yards away from the incident. The referee was right on top of it. It was a diabolical decision.
"I didn't expect that sort of defensive concentration from them. I feared the worst after their 4-2 defeat by Everton on Sunday. I keep thinking that the Leicester fairytale can't continue, but the fans here believe.
"What I will say, however, is that Atletico might prefer playing Leicester at the King Power where they will be forced to come out and attack."
'The tie is still alive'
The Foxes, who are the last English side left in the competition, were on the back foot for most of the match in Spain and failed to register a shot on target.
However, they remain firmly in the tie going into the second leg, which takes place at the King Power Stadium on 18 April.
"I think 1-0, we would have taken that before the game," said Shakespeare, who took over as boss of the Premier League champions in February following the sacking of Claudio Ranieri. "We came to try and get the away goal but we have seen what a top team Atletico Madrid are.
"The message is that mentally and physically we have been in a game and have given a good account of ourselves.
"We have got a good record at the King Power and the tie is still alive."