Barcelona suffered a shock 1-0 home defeat to Rayo Vallecano that means Real Madrid can step up plans for a title party.
This was a third consecutive home loss for the Catalans, only the second time that Barcelona have suffered such a fate in their history, with Alvaro Garcia's early strike the difference between the teams.
It means Real Madrid's lead at the top is 15 points with five games remaining, putting Los Blancos on the brink of being crowned champions.
That will be confirmed if Carlo Ancelotti's team avoid defeat to Espanyol next Saturday, while Xavi's Barcelona have slackened their grip on second place with this setback.
Rayo Vallecano, sitting 14th in LaLiga before their arrival at Camp Nou, snatched a seventh-minute lead when Garcia made a smart run into space to meet a chipped pass from Isi Palazon and fired sharply into the bottom right corner.
Jordi Alba curled a shot that flicked wide off the post as Barcelona chased an equaliser, then Gavi wanted a penalty when he slumped to the turf under a nothing challenge from Santiago Comesana. Referee Isidro Diaz de Mera Escuderos was unimpressed and booked coach Xavi for taking his complaints too far.
A clever floated shot from Gavi hit the crossbar shortly before half-time, and although Ferran Torres lobbed the ball into the net from the rebound, he had been in an offside position.
Jordi Alba had a shot deflect over as Barcelona pushed early in the second half, but the home side were horribly flat, with Ferran Torres and Frenkie de Jong replaced by Memphis Depay and Nico Gonzalez as Xavi looked to inject some energy.
Depay had a close-range shot blocked by Nikola Maras, and the Dutch striker saw a stinging 20-yard strike punched over the bar by Stole Dimitrievski, but the equaliser would not come.
What does it mean? Barca on the slide
Barcelona had won their last seven home games against Rayo Vallecano in LaLiga, and were seeking eight in a row against them for the first time in the competition, but this was poor from them.
This is a bad time of the season to get stuck in a rut, and Camp Nou defeats to Cadiz in LaLiga and Eintracht Frankfurt in the Europa League have been followed by another aberration. Barcelona led the shot count 17-3 here, but despite playing 14 minutes of second-half stoppage time, they could not put produce a clinical finish.
New boys slay Camp Nou
Vallecano, who came up through the Segunda Division play-offs last term, have become the fourth promoted team to win both of their games against Barcelona in a LaLiga campaign, and the first since UD Salamanca in 1997-98.
They have also improved their record when leading at half-time in LaLiga this season to W8, D1, L0.
Famous night for Iraola
After his team's 1-0 win at home in October, Andoni Iraola has become the first head coach to beat Barcelona home and away in his first season in LaLiga since Quique Hernandez for Hercules in the 1996-97 campaign.
The home win in the autumn was a result that was followed by Barcelona sacking Ronald Koeman, leading to Xavi's appointment.
Barcelona will have home advantage against next Sunday when Real Mallorca visit Camp Nou.
Rayo Vallecano are also back on duty next Sunday, hosting Real Sociedad.