Pompey, some pounds 5m in debt and with all their players up for sale, earned their place in the fourth-round draw by virtue of the veteran striker Steve Claridge's first-half goal and as their 2,000 supporters celebrated a rare success in a grim season, Forest fans called for the dismissal of the manager Dave Bassett and the resignation of the plc board.
Less than two years after Nigel Wray, Irving Scholar and the present chief executive, Phil Soar, bought control at the City Ground promising a return to the success the club enjoyed under Brian Clough, Forest are bottom of the Premiership and looking relegation certainties after a record run of 17 matches without a victory.
Portsmouth are not exactly pulling up trees themselves - they have won only twice in their last 12 Nationwide League games - but Alan Ball's side were good value for their win.
Bassett, suffering from flu, declined to appear at the press conference afterwards, leaving his assistant, Mickey Adams, to accuse Forest players of "lacking passion for the fight".
"I think Dave and myself can hold up our heads in the knowledge that we are doing our best, but some of the players perhaps need to look at themselves," he said. "We are in a situation that demands a lot of character and there was not too much of that in evidence today.
"We have prepared them properly and the rest is down to them. The Portsmouth lads have seen their top players sold off, but have they laid down?"
Adams dismissed suggestions that Bassett would resign. "This may well be the toughest situation he has faced but I don't believe he has ever walked away from a job and I don't think he will now," he added.
Ball echoed Adams' criticism. "Players have got to have the responsibility and it's too easy to hide behind the manager when things are not going right," he said. "Players do not take as much of the blame as they ought to. I don't like it when these big stars drive off in their big cars and creep off into the night and don't take the flak that they should."
Forest, without the suspended Pierre van Hooijdonk and inconvenienced in their passing game by a pudding of a pitch, enjoyed plenty of possession but were rarely able to ask searching questions of an understrength Portsmouth defence lacking Andy Awford and Thomas Thorgersson.
Portsmouth possessed altogether more appetite for the challenge, with outstanding performances from Sammy Igoe and Jeff Peron in midfield and from Alan McLoughlin in a sweeper's role. Indeed, Igoe almost gave them a dream start with a dipping shot that Dave Beasant was at full stretch to palm against his crossbar.
And then there was Claridge, who twice scored match-winning goals at Wembley for Leicester and whom Ball described as an "honest journeyman professional who will never let his manager down".
Socks around his ankles in characteristic style, the 33-year-old striker jabbed home the decisive goal after 17 minutes, slotting the ball past Beasant after Peron had found the Premiership side's defenders backing off as he made a beeline down the middle.
Scot Gemmill, the best of a generally bad lot in Forest shirts, missed narrowly in the first half and brought a good save from the goalkeeper Alan Knight in the second but, his efforts apart, the closest Forest were to an equaliser was when the Portsmouth full-back Matthew Robinson almost sliced a clearance into his own net.Reuse content