The Catalans picked up a 4–0 Clasico win at the Bernabeu on Sunday.
Former Arsenal captain Pierre-Emerick Aubameyang scored twice as Barcelona went wild in the capital.
And further goals from Ronald Araujo and Ferran Torres sealed Real’s disastrous result in front of a disgruntled home crowd.
The La Liga leaders are still 12 points clear of their fierce rivals in the title race.
But questions will certainly be asked of Ancelotti, who also came under heavy fire when Real looked like they were close to leaving PSG in the Champions League before Karim Benzema inspired a miraculous late comeback.
The talismanic French striker missed last night’s defeat through injury.
And his absence was felt as Real’s bizarre diamond formation failed to do the trick, with Ancelotti’s half-time adjustment to a three-back hinge making things right. even worse.
The Italian boss now insists he should take responsibility for the heavy loss.
But he urged everyone at the club to ‘stay calm’ despite being totally outclassed by Xavi Hernandez’s resurgent Barca side.
Ancelotti said: “We started with a one-on-one at the back, but it wasn’t the night for that.
“I’m sorry for the fans but we have to look forward.
“I tried to push higher up the pitch and it didn’t work. It’s my fault.
“It’s difficult because it’s a Clasico loss and losing to Barcelona hurts.
“We have to stay calm and recover our injured players.
“We’re sorry, but we don’t have to make a drama. Everything went wrong and I told the players it was my fault.
“It’s a very hard blow to lose this way. We know we have a good lead at the top of the table.
“We can’t make drama out of this game — we have to move on.”