Nine minutes into stoppage time, South Park Reigate’s Dan Moody swept the ball into the net and sparked pandemonium from the hosts.
For Guernsey FC it was a potentially fatal blow in their bid for survival and symptomatic of a problem that has plagued them this season having dropped 15 points by conceding goals beyond the 85th minute.
The game was a six pointer, South Park sitting one spot above the Green Lions in safety, and the loss means a four point gap has now opened up with Guernsey having one game in hand.
It was a frantic opening half hour.
South Park took the lead through a Jack Turner free-kick on 24 minutes, but the Green Lions were back level after Ross Allen sunk a penalty won by Charlton Gauvain.
GFC were in front three minutes later, Gauvain heading home a Fun du Port cross.
They held on for only four minutes, a Craig McGee through ball sending in George Vincent to level things up.
The second half was nervous and scrappy, but ultimately decided late in injury time.
Guernsey FC play their first home game at the new Victoria Park stadium on Saturday against Westfield. Kick off is at 14:00.