Nelson were beaten by a last-gasp goal from Anfield-based club Lower Breck at Little Wembley on Saturday.
The Admirals had gone in at half-time 1-0 up thanks to a goal from Keiran Demaine after 25 minutes.
And that lead stayed intact until eight minutes from time when Jamie Henders levelled for the Liverpool side.
Worse was to come for Andy Harrison’s men as Leon Arnasalam struck the winner four minutes into injury time.
Harrison said: “Absolutely no words for what happened in that last 10 minutes.
“Completely unacceptable from us.”
Nelson were on top in the first half, although Lower Breck also had good chances.
Midway through the half, the Admirals went ahead as Demaine seized on a loose ball and fired in off the post.
Lower Breck responded well in the second half, pushing Nelson back, but the Blues defended well, getting blocks on everything the visitors threw at them.
However, with eight minutes remaining, Henders headed home from a clipped in ball from Paul Mooney after a spell of sustained pressure from Lower Breck.
Nelson also survived a one on one, as Arnasalam squandered an opportunity to win it for the Merseysiders.
And with Nelson counterin, the game was wide open.
In injury time, Lower Breck got the winner as Henders putting in a great ball for Arnasalam, who this time made no mistake, slotting home from a tight angle.
Nelson’s home game with Holker Old Boys on Tuesday was postponed and they host Garstang tomorrow and Carlisle City on Tuesday night.