South Shields suffered a setback in their promotion bid after falling to a 3-2 defeat at Bamber Bridge.
In a topsy-turvy game against the fourth-placed side, Shields conceded the vital goal in the closing stages as they missed out on the chance to return to the top of the table.
The Mariners started brightly, with Gavin Cogdon and Jamie Holmes both going close in the early stages.
Brig survived again when Robert Briggs curled an effort narrowly over from the edge of the box, before Shields made the breakthrough in the 24th minute.
Matty Pattison looped in a superb corner from the right and Jon Shaw met it with a bullet header at the back post as he netted via the crossbar.
Shields’ positive start continued as they attacked with real purpose, but they were stopped in their tracks when a penalty was controversially awarded against them in the 29th minute.
Julio Arca was adjudged to have fouled Regan Linney in the area, and Adam Dodd stepped up to convert from the spot as the hosts drew level.
Brig made it a quickfire double with a second goal in three minutes as Alistair Waddecar ran clear on the right before delivering an inch perfect cross onto the head of Adam Roscoe, who made no mistake from close range.
Holmes and Shaw had half opportunities to level before the break, but the Mariners went in a goal behind at half-time.
They started the second half with added purpose, though, and went close when a sweet strike from Briggs was well saved by Lloyd Rigby.
In the 51st minute, Shields were awarded a penalty for a push in the box, and Briggs rattled home the spot-kick to bring them back on level terms.
Cogdon almost made it 3-2 to the Mariners when he wriggled through from the right, but Rigby did well to keep out his low effort.
At the other end, Dillon Morse and Shaw kept Shields level with fine blocks as the home side attempted to respond.
Louis Storey, introduced as a substitute by Shields in place of the injured Pattison, almost scored with his first touch as Holmes swept the ball to him, but he agonisingly found the side netting.
Brig netted the winner with nine minutes remaining after Shields goalkeeper Liam Connell had made a fine save from a corner.
The ball was kept alive and Kieran Charnock scrambled it over the line to spark home celebrations.
The Mariners tried in vain to force their way back into it, with Finnigan heading the best opportunity wide after being picked out by substitute Lee Mason.
They will now look to get back to winning ways on Saturday when they host Ramsbottom United at Mariners Park.
SOUTH SHIELDS: Connell, Baxter, Lough, Shaw, Morse, Arca (c), Pattison (Storey 67), Briggs, Cogdon (Mason 75), Finnigan, Holmes. Subs not used: Woolston, Ross, Armstrong.
Goals: Shaw (24), Briggs (pen 52).