Party-pooping Boro put Southampton's Premier League return on ice as they stormed from behind to beat the Saints 2-1 at the Riverside.Southampton - backed by 3,000 travelling fans - needed a victory to all but seal a second successive promotion and it looked certain when Billy Sharp drilled in the opener early on.
But battling Boro clawed back into the game and levelled in first half stoppage time as Nicky Bailey fired in through a crowded box and the Teessiders grabbed a winner late on with a sizzling Merouane Zemmama free-kick.
Reading had drawn 2-2 at home to Crystal Palace earlier in the day but Southampton's defeat secured the title for the Berkshire side.
"It would have been nice to seal it today, especially for all those fans who have come such a long way to support us," said Southampton boss Adkins.
"But we will have to make sure we do it at home now. We know what we have to do next week when we play Coventry at our place in front of 32,000 people. We have to make sure.
"We had a great start here, it is not often you score a goal in the first minute and that should have given us a platform to go on and win the match, we know that.
"But credit to Middlesbrough, they worked very hard to get back in the match and they made it very difficult for us.
"We conceded a goal just before half-time and we never got the flow going again after that.
"Obviously we are disappointed but respect to the opposition because they worked very hard to get back into the game and because they set out to win it, both teams did. I think it was probably a good game to watch."
Boro would have been out of the play-off picture had they not taken all three points.
"That (win) keeps it alive for us, it keeps it bubbling," said Boro boss Tony Mowbray.
"We know now we have to go to Watford and win. Yes, we would still need someone else to do us a favour - Cardiff have a tough game at Crystal Palace - but we are still in there and fighting.
"And, to be fair, the lads deserve it after that performance. To come from a goal down so early showed great character.
"But they knew they had to. They knew that only three points would do and they played in a very positive manner.
"I just hope that the energy they showed and the atmosphere in the stadium gives us the little bit extra we need when we go into next week's game."
Southampton were ahead inside the first minute after a defensive mix-up in the Boro box.
Bailey and Stephen McManus got in each others way as a cross sailed in, leaving Rickie Lambert free to square and Sharp lashed home from 10 yards out.
Boro responded well and went close as both Bailey, Barry Robson and Justin Hoyte put efforts from distance over the bar.
Saints went close on 21 minutes as Richard Chaplow skipped over a McManus tackle to lash a 12 yard shot just over.
But Boro were back in it two minutes into first half injury time as a Joe Bennett cross was cleared and fell to Bailey 25 yards out and he sent a low effort skidding through the box and inside the far post.
A minute later Robson sent Lukas Jutkiewicz into the box to drill home but the offside flag had already gone up.
The second half was scrappy and feisty and tackles and yellow cards were flying about.
There was a moment of controversy after 55 minutes as Boro's Seb Hines - already on a yellow card - brought down Adam Lallana with a tackle from behind on the edge of the box.
Although referee Tony Bates gave a free-kick he did not give him Hines a second booking.
Boro went close after 70 minutes as Tony McMahon drilled in a low free-kick that saw keeper Kelvin Davis dive full length to push it behind.
And they grabbed the winner when they won another free-kick just outside the box after 78 minutes.
Moroccan midfielder Zemmama chipped the ball over the wall and beyond the keeper for the deciding goal.