The Robins needed to beat Bristol City to have any hope of avoiding the drop to League Two, and Aaron Brown's scorching left-foot shot briefly ignited their hopes.
They were in front for almost 20 minutes until a neat back-heel by David Noble put young David Cotterill in for City's equaliser.
Freddy Eastwood's brace sparked Southend's promotion celebrations at Swansea as the Shrimpers twice came behind at the Liberty Stadium.
Steve Tilson's side needed to match Brentford's result to make sure of their move up a division before next week's final round of matches.
And with the Bees drawing at home to Hartlepool, a goal in either half from their top scorer was enough to secure promotion to the Championship.
Rory Fallon had twice given the hosts the lead as they looked for the win to send them into the top six, but striker Eastwood's double means they are still to clinch a play-off spot.
Northampton won promotion from League Two with a 1-0 home win over Chester, Scott McGLeish got the winning goal.
It leaves Leyton Orient and Grimsby battling it out for the final automatic promotion place to League One.