Miguel Flano, Pierre Webo, Valmiro Valdo and a brace from substitute Ion Echaide Sola did the damage to leave the Seville club just one point above the bottom three following Celta Vigo's victory over Atletico Madrid.

Betis, who were also beaten 5-1 by Osasuna earlier in the campaign, must now get something from their finale at Racing Santander next week to be certain of top-flight football next season.

Despite the tension surrounding the Manuel Ruiz de Lopera stadium, Betis pushed forward from the whistle and Osasuna goalkeeper Juan Elia was called into action in the opening minutes to turn away a free-kick from David Odonkor.

Elia then had to show even greater agility after 13 minutes from another free-kick. Marcos Assuncao fired in a bullet from 35 yards which the visiting custodian somehow got a hand to and pushed around the post.

Betis continued to pose a threat from every set-piece, and six minutes later another fierce free-kick from Assuncao had Elia beaten but was kept out by the crossbar.

It seemed only a matter of time before the deadlock would be broken, but when a goal finally arrived after 31 minutes it was not from the boot of a Betis player.

Osasuna defender Miguel Flano took full advantage of his side's first opening of the match by heading in Valmiro Valdo's corner to stun the Betis faithful.

The hosts immediately began the search for an equaliser and Juan Melli almost restored parity two minutes later but his six-yard header from Fernando's corner went narrowly wide.

And Elia ended the first half in much the same way he began it with another good stop to deny Fernando's 12-yard strike.

Betis and Fernando continued to trouble the Osasuna defence after the interval and the midfielder went close again with a drilled shot from distance which also finished wide of the right post.

Elia was by far the busier of the two keepers, with another crucial save five minutes into the half with his fingers denying Assuncao's cross-shot.

But the visitors were also helped by some wasteful finishing from the Betis players, including Juanito who failed to hit the target with his right-footed drive.

And the chances came back to haunt the home side in the 62nd minute when Osasuna doubled their lead completely against the run of play.

A rare foray into the Betis half allowed Webo to carve through the home defence before sweeping home from six yards out.

This time Betis were unable to muster a response as the reality of another defeat began to dawn on the players.

Ten minutes later Osasuna heaped even more misery on their misfiring hosts, Valdo powering home a header from Nacho Monreal's free-kick.

And with the celebrations still in full swing at the away end, the visitors made it 4-0 in the 75th minute as Sola smashed an 18-yard drive into the bottom right corner.

Sola then rounded off an evening to remember for Osasuna - and a night to forget for Betis - with his second of the match two minutes from time.