Dan Butler’s mistake proved costly as Pompey’s winning run came to an end at Leyton Orient tonight.
Shaun Batt pounced on a short backpass to grab the only goal of the game at the Matchroom Stadium.
The 57th-minute mistake was harsh on the teenager, who had again had a promising game for his side.
But it was a steep learning curve for the left-back as Pompey aimed to hit their best run of form for a couple of years in east London.
Guy Whittingham’s side deserved a point and had the chances to get a return from the game.
Jed Wallace and David Connolly had a couple of efforts apiece but the visitors failed to hit the back the net.
Pompey’s travelling army of support were excellent all night and set the tempo with their vociferous backing.
And it was an encouraging enough start from the Blues, who were unchanged for the third game on the bounce.
Wallace was back to his busy best and a lung-bursting 40-yard run brought a shot from 25 yards, which Jamie Jones gathered at the second attempt.
Orient threatened from a corner but Scott Cuthbert headed well over the top.
But it was the visitors who were marginally edging it and Wallace should have put them in front after 22 minutes.
It was Patrick Agyemang who carved out the chance, charging into the box and squaring for the teenager – but his first time effort was kept out by Jones.
Orient had their best chance of the half six minutes before the break as Cook found space 25 yards out but Simon Eastwood superbly pushed his effort around the post.
Pompey started the second-half in the same confident mood they had the first.
Wallace was left nursing a sore head after a clash with Leon McSweeney but referee Gary Sutton saw nothing wrong with the incident.
The visitors’ hard work was undone after 57 minutes when Orient made the breakthrough.
And it was a nightmare for Butler as his backpass on halfway was intercepted by Batt.
The Orient striker gratefully accepted the gift and made no mistake in a tough moment for the Pompey youngster.
But Pompey rallied from that blow and created good chances to level.
Connolly found space in the box and his 64th-minute shot was on its way in until Elliot Omosuzi cleared off the line.
Jack Maloney, on as a second-half sub, then charged half the length the pitch and cut the ball back for Wallace but he scuffed his shot.
Then Connolly found some space out wide in the box but drove his angled drive inches wide.
Pompey pushed for the equaliser but a couple of late set-pieces came to nothing as they left the capital empty-handed.