It was an afternoon to forget for Pompey, as their unbeaten record finally came to an end.
A struggling Gillingham side were their conquerors at Fratton Park, with a stunning Tom Eaves strike followed by Alex Lacey’s goal from a corner just before the break.
Defeat for Peterborough, however, meant that the result failed to knock them off the top of the League One table.
But Kenny Jackett knows his troops will have to up their game if they are to stay there for a prolonged period of time.
The Blues boss said: “Gillingham did a very good job on us defensively in both halves and weren’t constructive enough.
“We had the lion’s share of possession, but didn’t do enough with it and couldn’t work their goalkeeper, despite getting into good positions.
"I was disappointed equally with both goals and it’s a mental thing. We switched off because we hadn’t been fluent enough at the other end."
“Lee Brown and Ronan Curtis got in behind the left side of their defence, but we weren’t able to get the final ball right and when it went across the box, we weren’t the ones on the end of it.
“The two centre-backs for them – Ehmer and Zakuani – were, like last season, excellent. They blocked everything in their 18-yard box.
“There were a lot of corners and we couldn’t get our head on anything, even though there was nothing wrong with the service.
“The first goal was an amazing strike, but he had a touch first and you have to get to it after that. You could see it coming.
“I was disappointed equally with both goals and it’s a mental thing. We switched off because we hadn’t been fluent enough at the other end.
“There’s no point sweeping this under the table. We have to learn from it because we didn’t do enough with the ball and were negated.”
David Wheeler was brought on at the break, but limped out of Fratton Park after being caught inside the box in the closing stages.
Jackett said: “Hopefully his ankle isn’t too bad. Whether it was a penalty or not is irrelevant now because nothing was given.
“I was pleased with his impact. He’s waited for his chance and did well when he came on, as did Ben Thompson.”