Boro’s automatic promotion hopes are fading fast and they are now three points behind second-placed Ipswich, who still have two games in hand.

Hellberg said it was “like watching paint dry” in the first half as Boro failed to turn possession into clear-cut chances.

Although the home side improved after the break, they were dealt a huge blow in the seventh minute of stoppage time when Andre Dozzell’s volley deflected in off Conor Chaplin to give Portsmouth all three points with what was their first shot on target.

Boro have spent much of the campaign in the top two but have lost form at the worst possible time, and next up is a trip to Ipswich next weekend.

“I’m disappointed, devastated,” said Hellberg, whose side have scored just four goals in their last six games.

“The performance was poor, actually. First half was terrible. It was like watching paint dry. I don’t know why, I must have failed with everything I told them before the game because we weren’t good enough.

“Second half, we come out better. Our best spell was in that first 15-20 minutes in the second half when we created enough to at least score the goal. But we are not efficient enough to do that.”

Hellberg, who said he could have taken all of his players off at half-time, added: “Again, we created most of the chances, but this performance was not good enough.

“It was boring, it was lazy, there was no energy, except in that first 15-20 minutes in the second half.

“In the first half, it’s slow, it’s boring, it’s no energy. It’s passive. That is the disappointment.

“But, I want to be clear, we should not lose this game. This was poor by the high bar we’ve set ourselves. We maybe don’t deserve to win, but you cannot lose this game when they have no chances in the whole game.”

Portsmouth boss John Mousinho admitted his side were “lucky” but praised the resilience of his players.

Pompey were set to drop into the Championship’s relegation zone until Chaplin’s goal, which ended an eight-game winless run.

Mousinho said: “It’s an amazing result at this stage of the season. It’s all about picking up points and we managed to do that.

“If you’re asking me about the result in isolation, we’ve come here to hopefully win the game and we’ve managed to do that in those circumstances, which was different to what we’ve been doing in recent weeks. It’s a brilliant three points for us.”

Mousinho added: “This Middlesbrough side are such a good team, they’ve been in the top two for 90% of the season.

“They have brilliant attacking players, brilliant defensive players as well. They’re so dangerous and that was part of the reason we set up as we did.

“We had to defend for long stages and get through some dark moments for the boys at the back.

“We did get lucky, let’s be honest we got lucky, but that’s something we just haven’t had this season. It tends to be the case when you’re down the bottom.”