The 27-year-old frontman was a pivotal figure as Alaves beat relegation from the Segunda Division by a single point last season.

In fact, his 25 goals saw him end the season as the division's top goalscorer, ahead of Murcia's Kike and Stefan Scepovic of Sporting Gijon.

A Bilbao statement on Friday read: "Athletic Club, Deportivo Alaves and Borja Viguera signed today, Friday June 13, an agreement so that the player can join our club .

"Viguera has signed a contract until June 30 , 2017 and his termination clause will be €30 million."

Viguera, who formerly played for Real Sociedad, is set to be unveiled by the club later on Friday.