Carling Cup winners Spurs suffered a hangover as they had seemingly failed to recover from their Wembley celebrations and they were never in contention against a Blues side which produced their best win of the season to boost their prospects of avoiding relegation.
It was their first win since Boxing Day and the first in nine games as they trampled Tottenham into defeat.
In the process Birmingham produced their first double over Spurs since the 1975-76 season and only their second at home since Alex McLeish took charge.
But it was Forssell, who has suffered a series of serious leg injuries over the last few seasons, who reproduced some of his best finishing to consign the visitors to a heavy defeat.
Spurs paid the penalty for trying to protect their team in preparation for their UEFA Cup game in midweek.
They started without the likes of Jonathan Woodgate, Ledley King, Robbie Keane, Jermaine Jenas and Aaron Lennon.
The plan quickly back-fired when Birmingham stormed into a sixth minute lead following a left-wing corner from Gary McSheffrey on his recall to the side.
Liam Ridgewell knocked on the ball to James McFadden who put over across which the alert Forssell headed home as the Tottenham defence was guilty of ball-watching.
Spurs never recovered from this shock set-back and they could consider themselves unlucky not to equalise when the casual Dimitar Berbatov smashed a curling shot against the post.
With Steed Malbranque attempting to get Spurs back into the game Birmingham's shaky defence had a few anxious moments but the game was sealed early in the second half when Seb Larsson scored direct from a free-kick.
Didier Zokora was the guilty player when he fouled David Murphy. Larsson curled his shot around Tottenham's defensive wall and into the top near corner of the net, well out of the reach of Paul Robinson.
Four minutes later Forssell, who had earlier been foiled from scoring by a brilliant save from Robinson, made no mistake after 58 minutes when he scored from a tight angle.
Larsson's cross was deflected by Zokora forcing Robinson to push the ball out to the feet of Forssell, who rammed it back into the net.
Tottenham were now all at sea and McFadden slipped a pass into the path of Forssell, who had no difficulty completing his well deserved hat-trick with a right-footed shot past Robinson.
Substitute Jenas popped up with a last-minute goal for Spurs, but it was only a token effort for a side living on the glory of their Wembley win last weekend.