In what is expected to be Andy Reid's last game for the club, Gary Megson's side took an early lead but were then forced on to the defensive for virtually the rest of the match.
Reid has been a target for Spurs for several months, but it is believed that Southampton have agreed a cash plus players offer for the 22-year-old.
Brett Ormerod and Darren Kenton are believed to be moving to Forest as part of the deal although the Saints have refused to confirm the deal.
The only goal of the match came in the tenth minute when Kris Commons sent Alan Rogers clear. Rogers beat the offside trap and crossed for Marlon King to fire a shot past Mark Tyler in the Posh goal.
But from then on the visitors, backed by a vociferous 4,000 travelling army, surged on to the attack and were extremely unfortunate not to grab an equaliser.
Dave Farrell was the star man for Barry Fry's side and he went closest to scoring when he hit the post after good play by Richard Logan and Sean St Ledger.
Forest created very little after the break although King did miss two good chances to extend the home side's lead.
As Forest hung on in the closing stages Megson appeared to clash with substitute David Johnson who was unhappy not to be brought into the action until the last two minutes.