England coach Andy Pick said if England cannot lift the quality of their play then they have little chance of lifting the ICC U/19 Cricket World Cup.
After watching his side sneak home by four runs against Associate side Ireland he said: "My nails are bitten down, they are non-existent.
"We keep winning and that is the best you can say and we have got players who are capable of winning matches against any other side.
"But not enough players are turning up and delivering consistent performances day after day and unless we start to do that then any idea we have of maybe winning the tournament will be academic," he added.
England's win sends them into the Super League along with Zimbabwe and their meeting on Friday will decide which of them tops Group D.
Ireland have one more group match, against Nepal on Thursday, before they will switch their focus to the Plate competition.
England's batting line-up fell foul of man of the match Niall McDarby, the Ireland fast bowler, who captured three wickets in an opening burst then came back to knock over the tail as he finished with 6-50, the best bowling figures of the tournament so far.
England slipped to 48-4 at one stage but were rescued by Rory Hamilton-Brown (55) and Ben Wright (50) who added 112 for the fifth wicket.
They collapsed again late on but their eventual total of 214 was just enough to win the match - but only just.
Gary Wilson's 69 from 98 balls seemed likely to see the Irish to a famous win as they reached 180-5 at one stage but the pressure of chasing a target in a must-win game just proved too much for their lower order.
Ireland wanted 10 runs from the final over and five to win from the last ball but Steven Mullaney (3-33) bowled Greg Thompson to send England into to last eight of the tournament.
Ireland captain Eoin Morgan said: "I can't believe it.
We lost it in the middle period when their spinners bowled quite well and we lost two wickets after a great start (they were 62-1 at one stage).
"Niall (McDarby's) performance today was outstanding. He has done that for us in the past and I hope he can do it for us in the future.
"Now we have to refocus as the opportunity is there to do well in the Plate after our last game against Nepal.
"We can't qualify now but we have lifted our game after our first match against Zimbabwe and we can do that again."
A minutes' silence was observed by the players and match officials before the start of play as a mark of respect for Irish umpire Trevor Henry, who passed away on Monday at the age of 51.
Henry was a member of the ICC's Associate and Affiliate umpiring panel and was selected to stand in this tournament before he fell ill.
The Ireland team also wore black armbands in memory of the official, who also stood in last year's ICC Trophy in Ireland.