Aritz Aduriz headed Athletic into a 26th minute lead from Mikel Rico's cross and the Basques survived a second-half Real fightback to dethrone the leaders on Saturday.
Karim Benzema and Cristiano Ronaldo were denied but Gareth Bale almost scored the goal of the season seven minutes from time when his shot from within his own half beat the goalkeeper but cannoned off the left-hand post.
Real can point to their injury list as excuses - particularly the lack of midfield creators Luka Modric and James Rodriguez, but boss Carlo Ancelotti will shoulder the blame in the coming week for surrendering their perch on top of the league.
Barça got a lucky break when Lionel Messi's 56th minute penalty was ordered to be retaken for encroachment after his initial effort had been saved, though the blaugrana did lose Daniel Alves to a straight red card ten minutes from time.
Messi bagged another hat-trick, his 32nd for his club and a Spanish football record, to take his goals to 30 for the season, level with Cristiano Ronaldo. Luis Suarez scored a brace to take his tally to seven, while Neymar netted to take him to 15.
87,151 watched the one-sided contest, the Camp Nou's biggest crowd this campaign.
Atletico Madrid drew 1:1 at home to Valencia to sit six points behind Real in third. Los Colchoneros led through Koke's 33rd minute opener but failed to net a second and had their lead hauled back by Shkodran Mustafi's 78 minute leveller.
Appropriately for a match in which eleven yellow cards were handed out, Valencia's Javi Fuego ('fire') saw red in the final minute.
Valencia are a point behind in fourth and then there is a five-point gap to Sevilla. The top four qualify for the Champions League, as in England.
With only a point between first and second, the showdown at the Camp Nou in a couple of weeks looms ever larger in deciding the outcome of this season's title.
The race to avoid relegation is a lot closer, with only three points separating Eibar in 13th from Almeria, third to bottom, in 18th. Andalucian clubs occupy the entire drop zone at present, with Cordoba and Granada propping up the division.
* The Copa del Rey final will be between Barcelona and Athletic Bilbao on the 30th of May.
Barca won 3-1 at Villareal to go through 6-2 on aggregate and Athletic won 2-0 at Espanyol to advance 3-1 and end hopes of an all-Catalan final.
After Villareal's draw at the Bernabeu there had been some talk they would claw back the two-goal deficit from the first leg but hopes were dashed only three minutes in as Neymar scored. Jonathan Dos Santos equalised before half-time but second-half goals from Suarez and Neymar again sealed the tie.
Villareal midfielder Tomas Pina seemed to be on a mission to maim however. Sergio Busquets was stretchered off in the first half after meeting Pina's flying studs and a horror tackle almost did the same to Neymar in the second, for which Pina was shown a straight red,
In Barcelona, Athletic raced into a 2-0 lead before half-time through Aduriz and Etxeita and did not let Espanyol back into the game.
Oddly, the Spanish football federation has not decided on a venue for the final yet. Real Madrid are not keen on hosting it, probably because the sight of Barcelona triumphing in their home stadium is too much to bear. Valencia's Mestalla looks likely to step in.
* Cristiano Ronaldo has now scored 30 goals in five consecutive seasons in a major league, which is a record, but given the cannon-fodder Real Madrid are up against most weeks one has to wonder how amazing that stat really is. Messi's 32nd hat-trick for Barça has equally failed to amaze for the same reason.
* Spanish football's Antiviolence Commission issued several fines this week as La Liga strives to clean up its act. Granada were fined €7,500 for failing to stop pitch invaders on two occasions and Sevilla the same amount for two unpleasant fan placards and one fan bringing in a can of beer.
Other fines included €1000 for an Eibar fan who insulted the referee, €300 to three different teenage Granada fans who invaded the pitch and €1,500 for an Athletic Bilbao fan wearing a t-shirt with insulting messages about the club's owners.
* Carles Rexach, the rent-a-quote former Barcelona player and coach who hangs around behind the scenes at the Camp Nou, is at it again. After spouting his belief that Lionel Messi ate too many pizzas last season, he has now claimed Real Madrid buy players on looks.
"Perez is selling an international brand," he explained. "Vicente del Bosque and Di Maria are too ugly for Real Madrid. They are different from Toni Kroos and James Rodriguez."
* Barcelona's youth system is falling to pieces, or its building at least. In a court battle over payments between the club and its contractor, it was revealed "two, four and even six pieces of glass 1m x 2m" came crashing down from the façade, potentially endangering pedestrians.
* In this week's rumours, Raphael Varane has again been linked with Man Utd. This time the figure is around the £30 million mark for the Real Madrid centre-back, who has missed training this week with a sore knee.
Meanwhile shopaholic Manchester City are said to be eyeing one of Madrid's bosses Carlo Ancelotti or Diego Simeone to replace Manuel Pellegrini following their loss to Barcelona.
Real midfielder Illara is attracting Liverpool according to rumours. The Basque-born 24 year-old has represented Spain up to U-23 level, while Man Utd may swoop for Barcelona's soon to be out of contract Dani Alves.
Los Merengues are also set to beat Barcelona and Man Utd to Porto's highly-rated right-back Danilo. A figure of €40 million has been mentioned.
French midfield star Paul Pogba has said he could leave Juventus this summer and would be interested in a move to Barcelona.
The Catalan giants are also sniffing around Bayer Leverkusen midfielder Lars Bender but face competition from Manchester United.
Finally Barça legend Xavi, who turned 35 in January, could ink a deal to play in Qatar next season in a three-year deal which could earn him €18 million. The former Spain international will still have a year left on his Camp Nou contract but is said to be considering an early getaway to the Middle East, having turned down MLS last summer.
This weeks' big games (kick-off times GMT):
TUE 10 MAR UEFA Champions League Round of 16 2nd leg:
Real Madrid (2) v (0) Schalke 04 19:45
THU 12 MAR UEFA Europa League Round of 16 1st leg
Villareal v Sevilla 19:05
FRI 13 MAR La Liga
Valencia v Deportivo La Coruna 19:45
SAT 14 MAR La Liga
Espanyol v Atletico Madrid 16:00
Eibar v Barcelona 18:00
SUN 15 MAR La Liga
Real Madrid v Levante 20:00
(c) Sean O'Conor & Soccerphile