Police in the English county of Hampshire are bracing themselves with at least 400 officers dispatched in the biggest soccer-related security operation in the area's history.
When the two clubs met last season at Portsmouth's Fratton Park there were 80 arrests and numerous ejections. Saturday's game starts at 12:30 p.m. a deal TV and police negotiated to slow pre-match binge drinking.
Police said most local bars and pubs have agreed to stay closed before the game.
Portsmouth chairman Milan Mandaric has been livid ever since Harry Redknapp walked out as his manager and joined rival Saints last month.
The tension surrounding Redknapp's first match against his old team was heightened by Nigel Quashie's 2.1 million-pound (US$3.9 million €3 million) move last week from Pompey to Southampton.
The deal prompted Southampton chairman Rupert Lowe to suggest Mandaric - a Serbian millionaire - has "run out of money."
"I have spent more on Portsmouth in petty cash than he has spent at Southampton Mandaric replied. "I respect Southampton as a football club but I do not have the same feeling for their chairman."
Mandaric was infuriated further when he was given "the cold shoulder" by Lowe at a meeting last week in London of Premier League chairmen.
With both clubs struggling to avoid relegation the FA fourth-round game Saturday is their most important of the season. It overshadows the other 13 matches on Saturday and two on Sunday.
In other key games on Saturday Arsenal is home against Wolverhampton Man United plays at Old Trafford against Middlesbrough and West Brom is home against Tottenham.
Also on Saturday: Blackburn vs. Colchester Brentford vs. Hartlepool Burnley vs. Bournemouth Charlton vs. Yeovil Derby vs. Fulham Everton vs. Sunderland Newcastle vs. Coventry Nottingham Forest vs. Peterborough Reading vs. Leicester and West Ham vs. Sheffield United.
On Sunday: Chelsea vs. Birmingham; Oldham vs. Bolton.
Portsmouth under new coach Velimir Zajec has lost three straight Premier League games and is only nine points above 18th-place Southampton. Saints' defender David Prutton is almost gleeful about Portsmouth's troubles.
"I'd really love it if they were dragged into the relegation fight he said. "Especially if we left them there. If they get dragged in the the more the merrier."
Both Redknapp and Zajec are trying to beef up their sides before the transfer window closes on Jan. 31. Redknapp was in Italy this week looking at Roma's Egyptian striker Milo who eventually went to Tottenham. He's also reported to be interested in Lazio's Goran Pandev.
A former director of soccer at Panathinaikos before taking over Portsmouth Zajec has been scouting players in Greece.
Fresh from beating Man United 2-1 on Wednesday to reach the League Cup final against Liverpool Premier League leader Chelsea remains in contention for four trophies - the two English cups the Premier League and Champions League.
Chelsea defeated Birmingham 1-0 in the Blues second league game of the season.
Defending champion Man United may have the most difficult draw in the fourth round. The Red Devils face 'Boro managed by former Man U assistant Steve McClaren.
Arsenal should have an easier time at home against Wolves.