Match ends, MK Dons 1, Brentford 4.
MK Dons' relegation from the Championship was confirmed when they were comfortably beaten by Brentford.
The Dons, who needed a win to keep alive faint hopes of survival in their first season in the second tier, were ahead when Nicky Maynard stabbed in.
But just three minutes later Sergi Canos curled in a superb equaliser.
Brentford's Lasse Vibe, Ryan Woods and Jake Bidwell struck in the second half to condemn Karl Robinson's side to League One football next year.
To have any hope of staying in the division, the Dons also needed at least one of Blackburn, Nottingham Forest, Fulham, Rotherham and Bristol City not to pick up a point on Saturday.
However, even when Maynard turned in a driven cross from the impressive Josh Murphy early on, they could not hold on to the lead for long.
Canos was allowed too much space on the edge of the area and struck past Cody Cropper, who has yet to record a clean sheet in 12 games this season.
Cropper was taken off at half-time with a shoulder injury, but replacement Charlie Burns - making only his second professional appearance - was beaten by Vibe and a superb strike from Ryan Woods.
MK Dons felt aggrieved when a late penalty claim was dismissed when Maynard appeared to be tugged back by Harlee Dean in the area, but soon after Bidwell whipped in a free-kick that evaded everyone to round off proceedings.
Robinson said at the start of the season that the Dons had a budget in the bottom three of the division, and, with only nine league wins, they join Charlton and Bolton in dropping to the third tier next term.
Meanwhile, Dean Smith's Bees have won five of their past six Championship matches as they move up to 10th in the table.
MK Dons manager Karl Robinson:
"I'm sick really. If I'm being honest, we've not been good enough all year,
"Some of them today have made my mind up for me for different reasons - I have learnt a lot that's for sure.
"It's a difficult one for us all to take and I want to say thank you to the fans, but the most honest thing for all of us is the league table.
"In terms of the football club we have to look at what we could have done differently in lots of areas of the football club.
"There will be a lot of talking going on in the next seven days for obvious reasons, but certain people will not be here next season."
Brentford manager Dean Smith:
"I thought the result was more satisfactory than the performance - I think we can play better than that really.
"But we have to credit MK Dons a little bit as well, they kept us honest defensively but I think the difference was that we scored the goals at the right times.
"We came here with a game plan and wanted to back up the rest of our performances in previous weeks.
"I've got a lot of respect for Karl Robinson and I think you can see the supporters here have respect for him too. He got a worthy round of applause at the end and it just shows the public here like him and they like the way his teams play."
- 22CropperSubstituted forBurnsat 45'minutes
- 16WalshSubstituted forKayat 45'minutes
- 23WilliamsSubstituted forRevellat 55'minutes
- 2ColinSubstituted forClarkeat 60'minutes
- 47CanosSubstituted forGogiaat 77'minutes
- 21VibeSubstituted forDjuricinat 60'minutes
- Andy Woolmer
- Shots on Target
Second Half ends, MK Dons 1, Brentford 4.
Offside, Brentford. Alan McCormack tries a through ball, but Alan McCormack is caught offside.
Corner, Brentford. Conceded by Charlie Burns.
Attempt saved. Akaki Gogia (Brentford) right footed shot from the right side of the box is saved in the top centre of the goal. Assisted by Konstantin Kerschbaumer.
Attempt missed. Ryan Woods (Brentford) right footed shot from outside the box misses to the right.
Goal! MK Dons 1, Brentford 4. Jake Bidwell (Brentford) from a free kick with a left footed shot to the top left corner.
Foul by Antony Kay (MK Dons).
Akaki Gogia (Brentford) wins a free kick on the right wing.
Attempt missed. Akaki Gogia (Brentford) left footed shot from outside the box is too high. Assisted by Josh Clarke.
Attempt missed. Marco Djuricin (Brentford) right footed shot from outside the box is close, but misses to the left.
Jake Forster-Caskey (MK Dons) wins a free kick on the right wing.
Foul by Konstantin Kerschbaumer (Brentford).
Foul by George Baldock (MK Dons).
Alan McCormack (Brentford) wins a free kick on the left wing.
Goal! MK Dons 1, Brentford 3. Ryan Woods (Brentford) right footed shot from outside the box to the top left corner. Assisted by Marco Djuricin.
Substitution, Brentford. Akaki Gogia replaces Sergi Canos.
Hand ball by Ryan Woods (Brentford).
Foul by Jake Forster-Caskey (MK Dons).
Alan McCormack (Brentford) wins a free kick in the defensive half.