Barca were again without the injured Lionel Messi, who is not expected back until mid-January, and turned in probably their worst performance of the campaign against a fired-up Bilbao at the Basque club's new San Mames stadium. (Agencies)
A miserable week for the Spanish champions, which included Tuesday's 2-1 Champions League loss at Ajax Amsterdam, went from bad to worse when Bilbao's Iker Muniain turned in a Markel Susaeta cross in the 71st minute.
Barca barely mustered a shot on target against the Basques, with Brazil forward Neymar particularly disappointing, and it was only the second time in 15 matches this season they have dropped points after the 0-0 draw at Osasuna in October.
Gerardo Martino's side are level with Atletico on 40 points following the Madrid club's 2-0 win at promoted Elche on Saturday, while Real, whose 4-0 drubbing of Real Valladolid on Saturday included a hat-trick from record signing Gareth Bale, are three points further back in third.
Bilbao ended a streak of 14 winless La Liga games against Barca and beat them for the first time since May 2006 to climb above Villarreal into fourth on 29 points.
"It was an open game against a great opponent who pressed us hard and knows how to play," Barca midfielder Sergio Busquets said in a pitchside interview with Spanish television broadcaster Canal Plus.
"They had their moments and we had our but if we don't take our chances then it becomes harder," added the Spain international.
"All we can do is congratulate Athletic and carry on because there is no other choice.
"We have to learn from the mistakes we made, which for sure were numerous, but also from the positives."
Without World Player of the Year Messi creating havoc in the forward line, Barca lacked a cutting edge against a well-organised Bilbao unit.
Neymar forced a superb save from home goalkeeper Gorka Iraizoz in the 12th minute but the Basque side refused to allow the Barca players time and space, creating several clear chances in the first period.
They came close when a deflected cross fell to Muniain and he scuffed his close-range shot straight at goalkeeper Jose Manuel Pinto, playing in place of the injured Victor Valdes.
Cesc Fabregas played Neymar through five minutes after the break but the Brazil forward, who had to change his boots in the first half after slipping over several times, sliced his effort well wide.
Bilbao, who thoroughly deserved their victory and are unbeaten at home this season, almost doubled their lead four minutes after Muniain's goal when Pinto produced a brilliant save from Gaizka Toquero's header.
Martino threw on Pedro late on but the Spain forward made little impact and Bilbao comfortably held on to their lead.
In the earlier kickoffs, pressure eased on Valencia coach Miroslav Djukic when a quickfire hat-trick from forward Jonas gave his erratic side a 3-0 win at home to 10-man Osasuna.
Osasuna had their 38-year-old captain Patxi Punal sent off in the 16th minute for a dangerous late tackle on Dani Parejo before Jonas struck from close range just before the break.
The Brazilian netted his second three minutes into the second half and made it 3-0 in the 53rd minute as Valencia moved up to ninth on 20 points.
Sevilla kept in touch with the European qualification places when they won 2-1 at Andalusian rivals Granada and tightened their grip on eighth with 22 points.
Sevilla's city rivals Real Betis drew 2-2 at home to fellow strugglers Rayo Vallecano and stay rooted to the bottom.
Barca were again without the injured Lionel Messi, who is not expected back until mid-January, and turned in probably their worst performance of the campaign against a fired-up Bilbao at the Basque club's new San Mames stadium.