Charlton's relegation into League One was confirmed after they were held by Blackpool and saw results elsewhere in the division go against them.
The Addicks were on course to secure the win they needed to prolong their survival win after Deon Burton and Jonjo Shelvey put them 2-0 up.
However, DJ Campbell scored a penalty for Blackpool after he had himself been fouled by Mark Hudson.
Substitute Lee Hughes equalised in injury time to break Addicks' hearts.
Charlton started the day 12 points adrift of safety and knowing that only a victory would prolong their Championship status for at least a few more days.
They did little to ease the nerves of the home support during the first half when, despite dominating, failed to make the breakthrough.
However, early in the second half Burton met Lloyd Sam's cross with a deft header that looped in to give them the lead.
They sent the Valley faithful into raptures 90 seconds later when Zheng Zhi fed the excellent 17-year-old midfielder Shelvey who fired an angled shot past Blackpool keeper Paul Rachubka.
Athletic's nerves were jangling again when Campbell slotted away his penalty, sending keeper Rob Eliot the wrong way.
Their worst fears were confirmed in stoppage time when Claus Jorgensen played in Hughes, who made no mistake with a curler from 12 yards to condemn the Addicks to League One football next season.
Charlton manger Phil Parkinson:
"This club has had low moments before and has come back, and it will do so again.
"Everyone here today will have seen the terrific unity from the supporters and also seen that we have got some quality footballers.
"I don't want to discuss my position because that would be disrespectful to everybody at the club.
"It's not about me, it's about Charlton Football Club - a fantastic club which has been relegated but will come back strongly."
Blackpool caretaker manager Tony Parkes: "We're not safe yet so we will stay focused and carry on.
"Charlton must have had problems here because they shouldn't be going down, they've got better players than a lot of teams around the bottom of this division.
"Hopefully they can get back. It's not always easy but if they can keep their players together they'll have a good chance."
E-mail this to a friend 
Printable version 
Bookmark with:
What are these?