Jay Stansfield pulled a goal back moments after being sent on as a substitute by Rooney, but Adam Armstrong's eighth goal of the season - five minutes from time - ensured it was 14 points from the last 18 for Saints.

Luck also deserted Rooney, for whom defeat followed losses against Middlesbrough and Hull, with referee David Webb ignoring a loud penalty appeal after Southampton keeper Gavin Bazunu cleaned out Oliver Burke.

Rooney can take heart from the plight of Saints boss Russell Martin, who was himself under pressure after a run of four straight defeats in September but his side are now in the thick of the play-off positions after four wins from their last six games.

The Manchester United and England legend has said he wants to reshape Birmingham's culture to a possession-based team, and for the opening 45 minutes Martin's men provided him with the perfect template.

Harwood-Bellis headed in Adam Armstrong's cross following a short corner routine to give the hosts a perfect start, with the assistant failing to notice the Manchester City loanee was in front of the last defender.

The visitors looked bedraggled and beaten when slick Saints opened them up again on 22 minutes, Kamaldeen Sulemana playing a one-two with Adam Armstrong and crossing for Argentine Alcaraz to tap in at the far post.

The home side seemed to be in total control but when Krystian Bielik lofted a pass for Burke, he got his head to the ball first and was smashed to the ground by Bazunu's rash challenge.

Rooney raged on the touchline but referee Webb dismissed the strong appeals.

Adam Armstrong twice went close to extending the lead, while Stuart Armstrong's weak finish also ensured the visitors were still in the running.

Rooney sent on 20-year-old Fulham loanee Stansfield and he pulled a goal back with his second touch. It was a great finish but the build-up was, ironically, pure route one as goalkeeper John Ruddy punted the ball long and Lukas Jutkiewicz flicked on for Stansfield to expertly find the top corner.

The complexion of the game changed and a Saints defence that has conceded more goals than winless bottom club Sheffield Wednesday, suddenly looked uncertain, as Jutkiewicz shot straight at Bazunu, with the unmarked Stansfield screaming for the ball at the back post.

Southampton gradually reasserted control and substitutes Ryan Fraser and Samuel Edozie combined to set up the killer third goal, the former crossing for the latter to head the ball down for Adam Armstrong's eighth goal in nine appearances against Birmingham.

Southampton manager Russell Martin told BBC Radio Solent:

"I said at half-time we were really brilliant. I watched with a lot of pride, some incredible football. But there were some moments of sloppiness as well, which I was frustrated with.

"It's my job to be frustrated, and be relentless with them, because of the talent we have in there.

"They didn't start the second half very well, so we need to work that out, and we wrestled back control and had a few good moments but their goal came out of absolutely nowhere which gave them a spell and for 10 minutes we wobbled a little bit.

"Thankfully the subs came on, had a real impact and helped us wrestle back control of the game and we got the third goal which made it a nice last 10 minutes for everyone.

"We deserved to win. We gave them so little before that goal came out of nowhere. I'm annoyed that we didn't keep a clean sheet because people can level [the accusation] that we're open, and we weren't at all, today."

Birmingham manager Wayne Rooney told BBC Radio WM:

"I'm obviously disappointed we've lost but there were a lot more positives today.

"I told the lads before the game Southampton are the best in the league with the ball so we knew they'd have it quite a bit, and our organisation and shape was important. We did that well.

"In the second half we were better. I wanted us to play more in their half and pin them in, and we did that at times. But the goals we gave away were sloppy goals, one from second phase at a set-piece, and two from not marking at the back post. Those details need to improve.

"The [lack of a] penalty is an absolute disgrace. I have no other words to use on that because everyone in the stadium can see the keeper's running out, he's reckless, and if he comes out that quick he has to get the ball. Olly [Burke] flicks it on, and he wipes him out.

"I spoke to the fourth official and he said the referee was clear there was minimum contact and he was unwilling to take advice from his linesman or the fourth official. That's a big disappointment because sometimes referees do need help and I'd like to know how he was so clear that wasn't a penalty."