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Yang Min-hyeok’s 23rd-minute strike separated the two sides as Boro’s unbeaten league start came to an end, while Portsmouth snapped a four-game winless run to move up to 13th on the table.

Coventry's earlier thrashing of Sheffield Wednesday had put the pressure on Boro to reclaim first place in the division, but it buckled away from home.

"It was frustrating in the end. I'm gutted for the fans that we're not able to take something away and they enjoy the journey back," Boro boss Rob Edwards said.

"It's been a really good start. Today was frustrating, and we have to take it on the chin and continue with the work that we're doing that is building something.

"We had more shots than them, but not enough, and not enough on target, certainly.

"Once they got the goal, which was obviously avoidable from our point of view, it gave them something to hold on to as well. We weren't quite good enough in the final third, ultimately.

"We chucked as many forwards on the pitch as we could to try and get something, but it didn't happen."