"I have resigned, but I'm not prepared to talk about it. You will have to ask the board," said Francis, who was unaware of the incident until his wife, Helen, and son, Matthew, told him.
Francis and Birmingham's players reportedly were unhappy about the area being handed over for hospitality as it is usually reserved for use by players, wives and families.
David Gold, the chairman who is one of three owners of Birmingham, described Francis's decision as a "tragedy", and said: "On behalf of the board, Birmingham City and the fans I apologise to Trevor, his wife Helen and son Matthew for this unfortunate event. I hope they can find it in their hearts to forget this very unpleasant incident and return to the fold."
If Birmingham cannot persuade Francis to reconsider his decision, Steve Bruce, the former Manchester United captain who is now a player at St Andrews, and Brian Little would be leading contenders to take over.
Bruce said last night: "It's a big shock. I do not know what has happened but Trevor is very much a man of principle." Little resigned as Aston Villa manager 11 days ago but has insisted that he would return to the game at some stage.
The incident soured an otherwise good day for Birmingham, who beat Queen's Park Rangers 1-0 with a goal by Dele Adebola to move into seventh place in the First Division.
Sunderland will be looking forward to a return to the top flight after another thumping win, 4-1 over Stockport, sparked in the Stadium Of Light by a Niall Quinn hat-trick. "He struggled with injuries when he first came here but he has proved what he can do," the Sunderland manager, Peter Reid, said of his former Arsenal striker. "I am delighted he has got his rewards."
One unwanted reward was an injury and Quinn limped off. "His ankle is in ice-packs. We will know more in the next 24 hours whether he will be available for Tuesday night [when Sunderland play Birmingham]," Reid said. The three points lifted them above Middlesbrough into the First Division's automatic promotion places for the first time. "We know we have got to keep churning it out until the end of the season - I just wish it was May now."
Speaking of Brum, what moneyed supporters they have. One lobbed a mobile phone at a linesman in the second half of their defeat of Queen's Park Rangers at St Andrews. The incident - was it a bribe or an assault? - will be included in the match official's report, according to Francis. Adebola's header, his third goal in three games, keeps Birmingham lurking on the play-off fringes.
Two other contenders met at Portman Road. Charlton Athletic led Ipswich Town from the 22nd minute thanks to Clive Mendonca's 19th goal of the season but the in-form East Anglian hosts recovered to win 3-1.
Falling like lead, Stoke City have now gone 10 games without a win and hit rock-bottom yesterday when they lost 2-1 to Huddersfield Town at the Britannia Stadium. The win lifted the Terriers above the Potters and out of the relegation places.
Port Vale moved off the bottom when they won 3-0 at Reading, a defeat which left the Royals down among the drop squad. Three hundred supporters invaded the Elm Park pitch after the match. "I won't react to mob rule," the chairman, John Madejski, said, while promising a "full investigation". "There's no point in making changes for the sake of it. We have to analyse what's gone wrong in a professional way and put it right."Reuse content