Clark was later penalised when he punched clear and collided with Mullen but the Saints goalkeeper saved Charlie Adam's 75th-minute spot-kick.

Referee Craig Napier disallowed the goal on the advice of assistant Jonathan Bell and McPake said: "I have seen it from three different angles and I have not changed my mind. He has guessed, in my opinion.

"Credit to him if he's got it right, what a call it is if he's got it right. But how he can see from the three angles I've seen that Danny Mullen is offside.

"Initially they didn't give offside, they gave a free-kick for a foul on the goalkeeper, and quickly changed their mind. I am just being told this but it's a different signal with the flag when someone is offside. He stands and holds his flag up and waves. He sees the foul, the linesman, and then they have changed that to offside.

"It's not why we lost the football game but in my opinion we scored a perfectly good goal. If it's an honest mistake, I can accept, but what I can't accept is people guessing.

"We are very unlucky not to be in the next round. That's credit to St Johnstone but I'm proud of my players."

Saints boss Callum Davidson expected Adam to equalise from the spot.

"They have a Premiership squad in my opinion so I knew it was going to be a really tough game with the likes of Charlie Adam, who I worked with down at Stoke," Davidson said. "He is a top-class player.

"I will look back at the penalty, I didn't really know what it was for at the time.

"I thought Charlie would score because I have watched him take penalties enough times. So it's a fantastic save."

Davidson added: "The lovely weather dried the pitch out a bit and made it quite difficult for both teams to play any sort of football, so I'm delighted to be in the next round."