Pompey Academy suffered a disappointing day in Southend, as they fell to a heavy 5-1 defeat in Merit League One.
Henry Sandat, Tayo Adedoja and Brian Garzon all found the target to give the hosts a comfortable lead at the break.
Mo Amir then extended United’s advantage further before Harvey Laidlaw curled home a free-kick to reduce the deficit.
But it was the hosts who had the final say, with Sandat finding the net late on to dent the Blues’ title hopes.
Professional development phase coach Liam Daish said: “It was a difficult game for us. We defended poorly in the first half and should have dealt with some of their threats a lot better.
"We had a lot of possession and entries into their box, but weren’t able to make that important first contact with the ball."
“Our front four were dominated by their back-line, while we never looked comfortable against their forwards, who were honest, willing, aggressive and physical.
“We came out after the break and were finally able to identify the type of game we were in. We were a lot more positive in our play.
“But we conceded a couple more goals that Toby would, on reflection, think he could have done better with.
“We had a lot of possession and entries into their box, but weren’t able to make that important first contact with the ball and Harvey’s free-kick was all we had to show for our efforts.
“Southend were much better at being able to capitalise when they got into our area – they had five chances and took all of them.”
Pompey: Steward; Whitaker, Vincent (Trialist 62), Fox, Laidlaw; Jewitt-White, Aston, Hurst; Kamavuako, Trialist, Howell