Until that point, Boro were much the better side in the second half but a lengthy delay for a fan taken ill halted their momentum, and moments after the restart, Gallagher struck.

Matt Crooks had a golden chance to equalise towards the end of 12 minutes of stoppage time but headed a free-kick from the right over the bar from inside the six-yard box.

Midfielder Crooks should also have scored with an earlier header from eight yards out, only for Lewis Travis to head off the line, and his Boro team-mate Aaron Connolly saw a right-footed shot deflected just wide after a slick move down the right.

It was just a second defeat for Chris Wilder as Middlesbrough manager since his appointment in November, as his side missed the chance to climb into the play-off places.

They also missed opportunities to clear their lines before conceding what proved to be the winning goal.

There was no obvious danger as a ball was played back towards the top of the Boro penalty area but McNair aimed a wild hack at it, made no significant contact, and it fell for former Southampton striker Gallagher to shoot across goalkeeper Joe Lumley into his right corner.

The visitors pressed for an equaliser in the closing stages of what proved to be an hour-long second period, but on-loan Brighton forward Connolly fired over from a tight angle before Crooks somehow missed the best chance of the night.

The extensive stoppage time was caused by an eight-minute period of treatment for a Blackburn supporter near the dugouts, midway through the second half, with the individual conscious as medical staff led them away on a stretcher.

Both sets of players took on liquids and tried to stay warm during an unexpected delay after half-time

Blackburn boss Tony Mowbray:

"Scoring the goal was always going to be difficult for us, without Ben Brereton and Joe Rothwell. But, although he's missed a fair whack of the season, Sam Gallagher's goals per starts is pretty good.

"Gallagher is fast, strong, good in the air, with a rocket in his right foot. I'm always encouraging him to hit it because when you watch it in training it's like a bazooka, his right foot. It's a great strike.

"He has all the attributes to be a top player. His ambition has to be to get to the Premier League. Hopefully with this club. Look at this monster of a boy who can run like the wind with a rocket in his boots. Why can't he play in the Premier League? He's working really hard and I'm delighted he's getting the rewards."

Middlesbrough manager Chris Wilder told BBC Radio Tees:

"I didn't enjoy watching my team's performance. There were so many things wrong. We started off on the back foot, they jumped on us and we were never bright enough or quick enough.

"We were in the ascendancy after half-time, we tightened up, we played quicker and pushed a good team back. That's what I was looking for from the beginning.

"We had two incredibly good chances but we didn't deserve anything out of the game."